20 miles one way? It is OK if you work 4 hours a day. Otherwise - your personal life will suffer, as well as doing any other sports or exercises. You can do it once or twice a week as a form of sport AND commuting (with a change of clothes of course and perhaps a shower too).
Distance is not a problem, just the time. Depending on wind, traffic lights, climbs etc, it can take from one hour (minimum, absolute minimum), to two hours. One way.