ELLE is taking part in the Sport Relief Swimathon. But swimming 5k burns hundreds of calories, so eating the right diet is crucial

Swimming. It's hard work. I used to think it was easy, doing leisurely laps up and down my local pool, revelling in my newly toned arms. But that changed when I decided to enter the Sainsbury's Sport Relief Swimaton. Turns out there's a big difference between swimming for 30 minutes a few times a week and attempting to do 5K in under 75 minutes.

Tiring as it may be however, I'm really enjoying my training. I'm following the Swimathon 5K Sub 1.5hr 10-week training plan, which involves swimming around 4K, three times a week. I'm burning around 1,000 calories a session (yes, you read that right. I am definitely having my cake and eating it).

Preparing for the race is both time consuming and physically (and mentally) draining. But I want to do well, so I shall persist. Which is why I consulted triathlete and sports coach Marsha El Hage, from sports coaching company RG Active, for advice.

Here are her top training tips, which are useful whatever you're in training for:

Keep hydrated during the 2-3 days leading up to your race. Carry a bottle of water everywhere you go and sip it throughout the day.

As well as having a balanced diet to keep physically fit, get plenty of rest to be mentally fit. If you have a morning race, get to bed early!

Eat easy-to-digest foods on race day: fruit, cereals, low-fat dairy products, steamed veg, lean red meat and greens are all good choices.

If your race is in the morning, eat a bowl of cereal, banana or energy bar. If it is in the afternoon, have a bigger breakfast and lighter lunch.

Avoid hard-to-digest foods: heavy grains, brown rice and foods that are high in unsaturated fats like butter and sauces.

For races longer than 1500m (where aid stations are available) make sure you have practiced ‘poolside sipping’ beforehand with energy drinks and water. Doing this while you train will help your body get used to them and be ready to perform.

Eat your pre-race meal 2-3 hours beforehand. This keeps your carbohydrate and protein levels at roughly 60%-40%, as the body uses carbs for energy during your race. A good pre-race meal would be a baked potato with tuna and sweetcorn. Beans or eggs on wholemeal toast can work just as well.

Smoothies are another good option if solid food does not appeal. You can be creative by adding protein powder with fruit to juices, water or milk alternatives (such as almond or soya).