There are two scenarios in which, despite my conditioning, I simply do not
feel strong. One is very simple: running. I can run 2-3 miles easily
(though not as fast as I'd like any more), but I have a lot of difficulty
running say 6 miles. My legs (especially my quads) just feel dead.
I have a similar experience climbing stairs (especially with a brief case)
and hiking up hill with a pack. Despite being strong in the "how much I can
lift" category and running several times a week, my legs quickly feel dead,
and when I climb, I have to move at a pretty slow pace (compared to most
people who I should be climbing circles around) in order not to hit the
wall. In other words, if I try to keep pace, it is a tremendous exertion, I
sweat like a dog, and I run out of gas. I've bonked several times.
I figure there's either something really wrong with my legs (couldn't guess
what it would be) or I'm not training right. (There is of course, getting
older. But given how much I exercise, I would expect to build up some
Alec, you have three energy systems. The Creatine Phosphate System, the Lactate System, and the Aerobic System. The CP System makes ATP very fast, but in small amounts. The ATP it makes comes from stored ATP in the muscle, the CP system can only function for 10 seconds at a time, and the whole system does not need oxygen, fat or sugar to function. The Lactate System makes more ATP than the CP system, but somewhat more slowly. Only sugar is burned to makt ATP, No oxygen is required, and several minutes of energy are the yield of this method of sugar burning. Finally, the Aerobic System make lots of ATP very slowly.Fat and sugar is burned to make the ATP, and the system is oxygen dependant. Endless energy is made available through this system.
Based on what you told me, you are probably training in the CP system and the lactate system. Even though you run three miles several times a week, you are probably working hard enought to be functioning in thelactate system. Because I know you workout program is functional,and I know what you do, I suspect that your weight training is closer to the CP system. Naturally, you feel like allthe work you do should translate. As a result, you need to work harder on the aerobic system. Rather than running 6 miles, try running 3.5. then 4, 4.5, 5, 5.5 and so on. Also, run slower, and add hills. You can check where you are training by measuring your heart rate. (220-your age times 85%) is the max heart rate you should achieve. In order to maximize your aerobic development, you should periodically sprint, but recover to sub-maximal levels while running.
Finally, bonking is a nutritional issue. You need to get fuel in your body before you exercise, and since you are efficient at using the fuel you eat, you need to refuel during intense activity. Also, since the muscles are mostly water, make sure you get plenty of water on board every day, more when you plan to exercise, and keep it comming while you are out on the trails.