It's interesting tht you point out how riding with a friend shortens the ride. You're completely correct. That fact is a function of the fact that cycling, like pretty much all forms of exercise, is usually not very interesting. The dirty little secret about exercising is that it is boring, it hurts, and it makes you tired. I don't particularly like exercising; however, I love having exercised. Doing it with someone alleviates, at least, the first of those problems.