if the pain continues after a few rides of similar length, then it may be a fit issue. i found that coming out of winter, i have a few aches and pains when i up the ride length but it goes away. this year i had bum pain and i realised that it was simply because my legs were getting too tired and not pedalling as much so less weight went onto the pedals and more into the sitting. this went away after a couple of rides.