Being the fat bastard that I am, I am the last person who should be commenting on this but...
Overall diet change will increase stamina noticeably after just a week or two. Eat less saturated fats, eat more high quality foods. Whole grains (not processed stuff... in general, if there's a choice between white and brown, pick brown. I.E. no white bread,
no white noodles, no white rice. Go for brown) & beans should provide most of your calories. Green vegetables and fruits next, and better if they can be eaten raw. If you aren't much of a vegetable person, just get a blender, throw in a bunch of raw carrots
and other veggies and some apples/bananas and force yourself to drink that as your daily dose of vegetables. It's a lot easier to drink a big glass of nastiness than to eat a plate of something you don't like.
For your protein and fat needs, eggs, soy products, beans, and dairy (yogurt and lower fat cheeses) are good choices. As far as meats, in general: seafood > white meat > red meat > processed meat for overall healthiness.
Eat a good breakfast, and split your other meals in half. If you are on a 9-5 schedule, split your lunch in half and eat the 1st half at 11am and the 2nd half at 2pm. Also, split your dinner in half and eat at 5pm and 7pm. This will boost your metabolism and
eating 5 small meals is better than 3 large meals.