When I started, 10 years ago, I felt "old" and I thought the 2.5 miles each way was plenty long enough. A few years later it was 21 round trip which I wasn't sure about but after awhile that also felt routine, pretty much no challenge. So don't be intimidated by the numbers people are throwing out. If you try some commutes, not necessarily every day but stick with it for a month or two, your perspective about the distance will change.