As others have noted, commuting will really pile on the miles. My Dad & I were both training for a 4-day self-supported bicycle tour. He's retired & got in a lot of longer rides. I commute to work and rode less than 10 miles a day, but every day. I was not able to find time for longer rides, and I was worried about that. But we were both in pretty good shape. He thinks because I was out there every day, and got the same # of miles in as him, just spread out over more days, that we had roughly equal training.

So commuting or a short daily ride can be very effective training.