I used to be quite a bit into the weight lifting scene, though you couldn't tell it much now due to my lack of exercise in the last 8 or so years. But my diet was about 75% protein which, if you buy correctly, will give you what you want. If you're looking for calories, use lean ground beef or ground buffalo. This will give you calories and protein. If you want leaner, but less calories, go ground turkey.
I'm thinking that if your workouts use so many calories, that you're talking cardio (hence marathon/triathlete), so you will probably want to go lean calories like turkey or in supplement form like Whey protein. Carb's are a good form of calories too, rice, pasta, etc ... this should help you with your energy for cardio workouts. Eat 6 meals a day, high carb ... probably than 40% carbs from pastas, but you want to keep a high protein diet to keep muscle, unless it is the look of an Ethiopian child on crack look like most marathoners
Running burns muscle as well if there is not enough internal fuel.
My weight lifting diet consisted of (in my weight gaining phases, which I cycled on and off on 6 week cycles to break it up):
4am: Nitrotech shake, Celltech shake
After lifting(5-6am): 12 eggs (but only 1 yolk due to cholesterol), usually scrambled with Tabasco sauce
9am:Protein shake and a 1/2 lb cold grnd beef meatball, 3 hard boiled eggs
Noon: Whole boneless/skinless chicken breast with rice(any flavor) and 3 hard boiled eggs
3pm: Protein shake and a 1/2 lb cold grnd beef meatball, 3 hard boiled eggs
6pm: Regular meal consisting of protein such as steak, burger, chicken, turkey, or fish with potatoes or rice with veggies
9pm: Protein shake
As you can see, this is high in calories and protein, which packed me up to 240 lbs of muscle at about 10%-12% body fat at 6'0". I'm not much the picture of good diet or exercise now at 225 lbs and about 20% body fat, but I can pack weight on anyone and can probably take it off fairly well, though not as well as a dietitian.