Where are you all when I ride?! I'm the only chubby cyclist I've ever seen on my local bike path.

I recently decided to try to get into good enough shape to ride a half century for my 50th birthday in March and I'm getting intimidated. I don't really know where to start. I've started riding my bike home from work (about 14 miles) 1 day a week, but I'm SO slow - it takes me an hour. I don't have much time to do long rides; I work nights/shift work, 1800-0600, so I have to ride alone at night on my days off, which limits areas and time due to safety, and I live in an extremely flat area so I can't practice hills. I'm getting in about 20 min rides as fast as I can at nights I don't work and I'm jogging on the treadmill for 30 min on the nights I do work (again, at an absymally slow rate but better than watching TV during break). I'm also chicken about riding in an event with my inexpensive bike, overweight, unmatching gear, and having no idea about the rules, but I'm going to suck that part up and just deal with it. It will be one day of my life with people I will probably never see again so I can get thru that part of it as long as I can finish it.

I don't know what my benchmarks should be so that I can judge if I'm getting in shape fast enough. I can't find information on line about a half (it doesn't sound like that's much of a goal for most people). Am I biting off too much? How do you know?