so you're doing centuries on a mountain bike?

maybe you should consider getting a more road oriented bike for that kind of riding ?

road bikes generally have 50T or 53T big rings. 39T or 42T is a common small (or middle if its a triple), although 50-34T is the standard for compact double fronts.

and yes, its all about spin on the road. power riding aka mashing is going to trash your knees, and wear out bike parts.