I read somewhere when I was starting, that barring any fit/comfort (or saddle) problems, anyone in decent shape should be able to do 20 miles easily if they don't push themselves too hard/fast. If you maintain just a 10mph pace, that's only 2 hours on the bike. 15mph, and that's an hour and 20 minutes... really not so impossible when you think about it in those terms.

Keep hydrated, and keep fueled, and you really should be just fine for however long/far you want to ride