Thanks everyone for the input. My son and I took all of your advice given and we're going to do a 52 mile ride this Friday and see how it goes. We're trying a route that'll put us on the 'Cross Florida Greenway'. I've never been on that ride, just a little worried about the terrain (it's unpaved). I think we'll do fine, but it's a definite step above what we've been doing. If I feel good at the end of that ride, I'll continue my plan to do the century in mid February. If it's all I can bear to get through it, I'll put it off until I build up my strength.
I honestly figured a fit issue would show up in 20 miles. I really hadn't thought about that. I'm hoping at least this 52 mile trip will give me an idea of how I'll fare on this bike and how 'ready' I am for 100 miles. If this trip works out alright, we have another one that's closer to 65 miles round trip that ends up in a really beautiful state park... but it also uses the 'Cross Florida Greenway' bike/multi-use path. Has anyone rode that trail before? It mentions on the website that although unpaved, it's for both casual and mountain bike enthusiasts. I'm just hoping it's not a sand trap to sink my touring tires in.