What is your past riding history? Were you doing intervals on the trainer, or just spinning along at minimum effort? For the 20 miles rides what is your average speed? Dehydrated, sick, not eating right recently?
I have met a lot of people who have decided to get in shape and spent the winter in the gym on a spin-bike. Different world out on the road and they get frustrated when they see what they can really do. If you are just starting out, it can take a long time to build up the endurance needed for this sport. Try and ride with them more often and the speed and power will happen.