IT'S NOT THE TRAINING THAT MAKES YOU FITTER!

Yes, of course you have to train, and to continue to improve your level of fitness you must at the right time train hard.

However, it's not the training that makes you fitter - it's your body's response to that training - so the body actually gets fitter between training sessions.  Many athletes make a great mistake by believing that all they have to do is train until it hurts, in order to get fitter and faster, BUT that's not the whole story:-

Training too hard, too often can result in injury, illness and chronic fatigue

SO: -

Have a balance of sessions in your programme

Do not schedule hard sessions on consecutive days

Get enough recovery between sessions (this may be active recovery such as easy running or an alternative activity, perhaps swimming)

Your body will respond best to a consistent lifestyle with a good sleep pattern

Make sure that you eat well (include plenty of fresh fruit and vegetables in your diet) and try to avoid junk food