It depends upon your level of health and fitness, but if you can run three miles easily, riding 26 ought to be doable. I could ride 26 when I was "untrained," I just couldn't do it very fast. My advice would be to get on the bike and ride a lot. Obviously, you want to be riding harder than you would if you were just touring around and admiring the scenery, but you don't want to work very hard right now. Moderate intensity - think about an easy jogging pace. If you're literally just starting out, you'll hear a lot about intervals and specific, structured training that just doesn't make much sense for a beginner. It's time on the bike that you need. I've been there myself, and you'd be amazed at what your fitness can become in six months compared to when you start.