For me to do that as a regular routine becomes, well, routine. @
coloso above either plays golf slower than me, or rides faster! Either way, I don't want to commit to 18 holes every weekend either. Sometimes I ride a metric, more often a "long" ride for me is 40-50 miles and it is rarely planned, it just happens and is dependent on how I feel 15 miles in, how motivated I am, the weather, and what else I want to get done on any particular day.
But no, it's not nuts to ride a metric every week if you want. Personally I believe (based on my own experience) that running is MUCH harder on the body, particularly the joints so I can't equate riding a metric with running a 1/2 marathon.