You should pay attention to your diet before and after a physical effort: as a matter of fact, we need energy before an effort, proper nutrition during the exercise and an adequate replenishment after the effort.
Before starting it’s good to take a lot of complex carbohydrates (cereals, pasta, rice, wholemeal bread); during the exercise you should often eat light food that can be easily assimilated.
In addition to complex carbohydrates you can take simple ones: the main foods to be taken during the effort are cakes, sweets, energy bars or specific sports drinks and bread rolls.
After the exercise there is the so-called “30 minute window”, that is half an hour when our body can utilize carbohydrates (especially if combined with proteins) at the best; thus a good supply at the end of a race is necessary: it would fit tuna or butter sandwiches, cakes, yogurt etc.
The day following the race (mostly in the case of a very hard effort) it’s absolutely necessary to take a lot of proteins to replenish muscles.
The last but not least advice deals with hydration: during a physical effort the main things our body requires are fluids.
Dehydration is very common, therefore you must drink at least one litre of water (better if you add mineral salts, too) every two hours.