Here is one for the experts...maybe this is common, but I don't know. Anyway, I ride with my son. He is 13 years old...when we do interval training together I am completely wasted at the end of the ride...by wasted I mean at the end of the interval my breathing is very labored. My son rides as hard as he can, but his breathing never gets labored. He says his legs start burning and that is what limits him..not his lungs. Is there any specific workout that can be done to improve in this area? We have gotten a book on weight training for cyclists...but I don't want too much of that since he is only 13 (going on 14). Any suggestions?? Thanks all!!