What kind of speeds are you talking about? 1 mph is more significant if it's 10 and 11 versus say 15 and 16. In any case there are a lot of things that could be in play... nutrition for one. Ever tried Sportlegs?
It could also just be random variation.
Thought two: you must ride those distances (i.e. times) back to back more often to get the endurance type adaptation you want. At first you will be tired in the second, third, fourth days of riding when training for that.
Last edited by billydonn; 02-28-09 at 07:45 PM.