How your body changes during training for your first marathon

Dr Juliet McGrattan talks through how your body changes whilst training for your first marathon.

You’ve signed up for your first marathon. You’re feeling a mixture of excitement and fear about what lies ahead. You’re looking forward to making this step up in your running and fitness but also wondering if your body is capable of it. You don’t want to be one of those runners who gets injured and has to pull out!

You will learn a lot about yourself during your marathon training. Your body will change and adapt as you work through your plan. Being well prepared will help you set yourself up for success. There are many things you can do to make your marathon training go smoothly. Let’s take a trip around the body, explore what changes you can expect to see in yourself as you head towards 26.2 miles and get some top tips for success.


Running is a great activity for cardiovascular fitness. As you progress through a marathon training plan, your strength and endurance will improve. This will be reflected in some simple measurements that you can track:

  • Resting heart rate – measure your heart rate first thing in the morning, before you get out of bed. See how many times your heart beats in 60 seconds. As you get fitter you will quite quickly see this number reducing, indicating that your heart is getting stronger and more efficient.
  • Blood pressure – regular running and strength training will lower both your systolic and diastolic blood pressures so after a few months you should start to see both numbers fall a little.
  • Heart rate variability – this is the time between heart beats and a fitter person will have more heart rate variability. Your smartwatch may measure your HRV. It’s affected by more variables than just physical fitness but it can be satisfying to see improvements.

If you’re interested in a heart rate guide including target heart rates during exercise, this is an interesting article. Remember though that everyone is different and there are lots of variables that affect your heart rate; standard calculations have their limitations.


Your lungs draw in oxygen from the air and transfer it to your blood stream so it can go directly to your muscles. When your muscles are working hard your breathing rate will increase to maximise the supply of oxygen, and to breathe out waste products too. Research states that as your body exercises, your breathing rate has to increase from about 15 breaths per minute (12 litres of air) when you are resting, up to about 40–60 breaths per minute (100 litres of air) during exercise. Your heart rate also has to increase to pump the oxygen-rich blood to your working muscles.

Healthy lungs have a large breathing reserve. This means that when you exercise it can make you feel breathless but that isn’t dangerous for you. This is not the same as being ‘out of breath’ which is an unpleasant sensation when you don’t have much breathing reserve.

You need to do a variety of different paced runs during your marathon training. Long slow runs where you can breathe comfortably, and faster interval runs where you can hardly get your breath. Over time you’ll notice that once you’ve warmed up and settled into a pace you don’t get as breathless as you used to. You won’t be gasping as much when you climb your regular hill, you can hold a conversation more easily on your slow runs and when you stop your breathing rate returns to normal more quickly too. Your lungs, heart and muscles all get more efficient.

Body weight

Many people think they will lose weight when they train for a marathon but in reality that’s rarely the case. There are two main reasons for this. Firstly, running makes you hungry! When your distances crank up to double digits the ‘runger’ (running hunger) can be intense. You might not feel starving when you get in from your run, in fact, your appetite hormones can be supressed. Later in the day or the following day however, you find yourself foraging through cupboards and opening the fridge looking for a quick fix. In the depths of hunger, you might not make the best choices. Training for a marathon can sometimes feel like a ticket to eat whatever you want and you might put on weight as a result. Remember, a well fuelled long run won’t necessarily leave you in a big calorie deficit.

Secondly, with your step up in training, you will be gaining muscle. This is particularly true if you’re doing strength work outs to support your running. An increasing muscle mass will give you a higher body weight. This is a good reason to ditch the scales as the numbers you see might not reflect how you feel.

Muscles and joints

You will quickly discover that you need to take good care of your muscles and joints when you’re marathon training. You will end up feeling stronger and your body physique may change but along the way you will experience muscle soreness, aches and general stiffness. These symptoms can be worse if you’re an older runner. You might get niggles and discomfort that you haven’t had before as you push your body to new limits. It can be tricky to know what’s a niggle and what’s an injury but with a marathon looming, it’s best to get advice from a physiotherapist or doctor early.


With regular exercise you might notice a healthy glow to your complexion but running in all weathers and temperatures can have a negative effect on your skin too. You might experience sunburn, dehydrated skin and reddening due to exposure to the elements. Sweat rashes are common too. Protect your skin from the sun with hats and sun creams, even on cloudy days. Use a rich moisturiser and lip balm to prevent skin drying out in harsh conditions. You’ll undoubtedly get some skin chafing as you extend your distances. Under sports bras, in armpits, around waist bands and between thighs are common areas for skin chafing. Experiment with different fits and fabrics for your running clothes and skin lubricants and barriers to protect hot spot areas.


Your feet are going to take a bit of a bashing during marathon training. You might notice blisters, red areas of skin and aching feet. Toe nails can become thickened through repeated trauma and you may even get a black toe nail after a very long run. It pays to look after your precious assets. A perfect shoe fit and decent well-fitting socks are your best bet. Alternating two pairs of trainers can be helpful to let shoes dry out properly between runs. Over moisturising your feet may not be in your best interest, a degree of skin hardening can be helpful.


It’s easy to assume marathon training is all about physical changes to run but you will also notice changes in your mental health too. Doing hard things makes you believe in yourself. As you move through your training plan you will grow in confidence. You will feel proud of yourself. Overcoming challenges, keeping going when you want to stop and completing runs that you didn’t think you could do all build incredible mental resilience. You will learn so much about yourself that you didn’t know.

Tips for making it to the marathon start line

Getting to the start line fit, well and ready to run is honestly the hardest part of marathon training. If you line up at the start, you will most likely finish. Some of the large city marathons have finish rates over 90 per cent. Making it to that start line is harder though. There’s a lot that can put you off track on your way there. It’s definitely not all about the running.  Thankfully there are plenty of things you can do to maximise your chances of making it successfully through your plan and reaching the start line. Here are some key areas to focus on:

  • Gradual training. Find a plan that suits you and stick to it. Large increases in distance, frequency or intensity of running put you at risk of injury. A good plan will bump things up gradually, step by step.
  • Recovery. It’s exciting to see your body change but don’t get carried away. Rest days are a vital part of training. Your body needs to adapt to the load you are putting on it.
  • Sleep. Your body strengthens and restores itself during sleep. It’s a good idea to aim for a little extra sleep each night during your training.
  • Nutrition. It’s often during your first marathon training that you become really aware of the importance of fuelling your body properly. Once you get beyond 90 minutes of running you will need to take on fuel while you run. Now’s the time to test out different fuels and find one that doesn’t upset your stomach, give you cramps or make you feel bloated. It’s also important to look at what you eat before and after you run. Your body needs carbohydrates for energy so don’t skimp on these. Protein is also vital, it’s not just for gym goers. Protein is needed to repair your muscle fibres and strengthen your body. The recommended protein intake for those with an active lifestyle is 1.2–1.4 g protein per kg of bodyweight per day with some research even advising up to 1.83g per kg per day. Getting into the habit of taking over 20g of protein and some carbs after exercise is also worthwhile in order to kickstart muscle protein synthesis. Plus, it’s worth investing in collagen if it’s not naturally in your diet as this helps strengthen your joints and ligaments.
  • Strength work. It’s possible to get through a marathon without doing any strength work but there’s more chance you will succeed and you’ll also find it easier if you do! Strengthen muscles to give you endurance, speed and to support and protect your joints. Make it part of your plan from the beginning and you’ll be standing tall and strong when you cross the finish line.
  • Stress levels. Marathon training is a team game. It can quickly take over your life, especially when those long runs get really long. While your mental resilience can improve during your training it pays to get as much support and help as you can outside of your running, especially around race day. What can you delegate? What favours can you call in? Who will support you? Find your cheerleaders early. Your success will inspire them and give them satisfaction too.

A first marathon is always scary but knowing what to expect can help you a lot. With the right expectations, training and mindset you can achieve it. Enjoy the way your body changes through your training. See yourself get stronger, fitter and healthier. You will never forget your first marathon!