If I may add to the experience of the offr who has written a first hand account.
I would put the physical fit requirement of trekkers as follows:-
1. Should not be carrying extra body wt ie should be slim.
2. More important than the physical str, the trekker should have strong lungs ie good lung capacity.
3. To be able to walk with a rhythm which is synchronized with your breathing. Never let yourself to break into panting.
5. Eat light food preferably dal bhaat & avoid non veg.
6. Never over or under clad yourself. There is very fine balance which comes from experience or talking to others who have trekked in high altitude.