Think there are so many factors, I struggle to do my 6 mile commute 5 days a week, my trip to work is easy and takes around 23 mins almost all downhill and a drop of 370m, my trip home takes around 50 minutes and I then have to climb back up that 370m, I generally try and take it easy as even one day would break me if i forced it. If those 6 miles were flat I would have no issue doing it every day.