From a couple previous threads I have read on the subject, the concensus seems to be that 40 miles round trip is a limit for a full work day. Mine is 14 miles one way, often extended in the nice weather, and I take a train home at night (I have a long workday, that I can't seem to overcome in order to ride home). Personally, I think for the purpose of fitness, weight loss, etc., 10 miles one way daily at least is a minimum worthwhile to work up a sweat; FWIW.