Coward...

Try the class and make sure you take some liquids to ensure you are hydrated. I am much like you in that I have not got up early to hit spin class, but I have been in that gym since about 2 weeks after surgery at least 2-3 times a week and for 45-90 minutes.  Last week I read C25K and am attempting to incorporate that training into the other 3 days a week that I exercise along with a little light weight training on smaller muscle groups. 

An instuctor recognizes when a larger participant is there and generally works to get you to a level where you are moving and exercising to benefit your needs.  I had a trainer last spring and he pushed me and encouraged me and still gives me encouragement now even though I have not yet returned as his client. 

A group or structured activity with someone is a great way to push oneself when you are first getting into the routine. Good luck tomorrow and maybe someday I will hit up a spin class, for now I will keep to the x-trainer for 30 minutes and treadmill for another 20 of course that is after my large muscle groups for 20-30 minutes.  I enjoy it when I am there, just getting there is the hardest part of the workout lol!!!!