I've got an event coming up in about 8 weeks, and I'm trying to do some workouts that will best help me acheive increased speed over realtively short distances (15-25miles, rolling hills). Here's my question, and feel free to flame this one - given the short time frame to train, what would be more effective?

A. Group rides that average around 30 miles - simply focus on pushing harder each week.
B. Spin classes that are much shorter in comparison (only an hour), but really push to the limit.
(I would still ride the bike, just not as much)
C. A better option which I haven't thought of, but some BF member with more experience will share?

I await your collective wisdom...