i ride like 30-40 miles 3 times a week. but the problem is that the first like 4-6 miles my legs just burn and i get tired and just want to quit but after that it is like i sometimes get a second wind i guess and i feel better and my legs don't hurt and then other times i just hurt to even get through 20 miles.it really sucks in races b/c i get dropped after like a lap and never can catch back up but after the race i am not tired at all. what is going on.