I agree with the post that it is my lungs that need the "warm up" time. My muscles come along OK. Until I get that second wind, as we called it back in the basketball days, I have a problem just keeping up with a group. I am not sure how it kicks in, but once I have that second wind I am good for a long time. Unfortunately I do not seem to have a normal amount of time or distance to get the lungs going as it seems to vary from ride to ride.