You will need a good diet of Protein, Fish, Steaks, Chicken (not fried crap) but dont have it fried... Grilled or Oven cooked and low(ish) carbs, Rice, Potatoes, Chips, Bread etc. Obviously you need to eat the carbs to keep body mass up!!. Work on building upper body stretgh AND lower body strength with body weight excercises (Pull-ups, Chin Ups, Sit Ups, Squats, Crunches, Lunges). When you go for your runs (assuming you do) break them up with various distances and chuck some sprinting in every now and then. The sprinting is what your going to need....