I agree with Maret - choose a tour suitable for your fitness. Of course fatigue can occur in any case. It can be due to several factors (bad day, dehydration...). But tours (especially longer ones) are a good way to lose a few kilos. Therefore I recommend drinking more instead of eating. On longer tours lasting several hours, isotonic drinks etc. are essential. Basic preparation of it (so you don't buy just anything) is a pinch of salt and a teaspoon of sugar. Good luck!