This pretty it. If you're just starting, and aren't in good shape to begin with, and it sounds as if you are not, you may be in for a shock if you try to cycle the 15 miles.

First time I commuted (12 miles) I was miserable, even though I was in pretty good aerobic condition having been a runner for a number of years. Granted the bike wasn't very good. The route I chose was worse. But you learn.

Don't spend too much time thinking about it. It's your first bike. Chances are if you get into this it won't be your last.