I do 21 miles per day. 6 on the way in using the train also, 15 on the way home (it does not matter how sweaty I am when I get home, my son still plays!). I tend to find that during the week my out and out speed drops, while my endurance seems to get better. At the end of the week I am normally slightly heavy legged, but nothing too serious. The 15 miles home including stops takes anything from 50 minutes to 1 hour depending on wind, traffic lights etc. My main concern would be that it will get really hard when you have a heavy cold / virus / misc ailment. In that case it gets harder to do the distance day after day. You will also need more batterys / better lights etc for the winter months....