While slower, weaker, and fatter than 99% of all riders on the Internet, my own experience is that commuting 20 miles a day, 3 to 5 days a week, 52 weeks a year, gives me a solid enough base that pretty much any recreational ride I want to do on the weekends is pretty straightforward. That would include long rides in the mountains, or centuries with 10,000+ feet of climbing. Again, I am almost certainly going to be the slowest, ugliest, and most Fredliest rider in the Century, but I will finish, and I will be back on my bike Monday morning.

As a former couch potato, my own experience is that commuting actually did get me in shape for more than commuting.