I thought this was going to be a thread about looking for longer routes to work so that you CAN legthen your commute.

Going 15 miles is no problem at all. By the most direct route, my commute is 10 miles. Often, especially if the whether is nice, I take a different route that makes it a bit over 13 miles. I'd go further if time were no issue, and I often head out at lunch just to enjoy some extra saddle time. It happens that my 13 mile route is on more open roads and usually doesn't take more than a couple minutes longer than the stop-and-go 10 mile route.

That said, you may have a brief adjustment period to get used to the extra mileage. Be sure to get proper rest when your legs are tired and it will be no trouble.