Two hours a day is not a crazy number at all for some endurance sports. If you do cycle racing 10 hours per week is a good starting point to build up endurance.

Two hours a day is not a crazy number at all for some endurance sports. If you do cycle racing 10 hours per week is a good starting point to build up endurance. But that works because cycling isn't weight bearing, so after the initial exponential learning curve you can pretty much do as much of it as you have time and motivation. 20 miles a day of running, on the other hand, is absolutely not typical. Other than ultra-runners no professional does >100mi/wk regularly simply because the diminishing returns don't justify the linear increase in injury risk and aggravation (it's running, injury incidence is already crazy high).