It's winter. It's a lousy season for me to go mountain biking. I don't get home from work till it's dark, and I don't live close enough to any decent trails that I could ride anyway, I have to drive a ways to get to the good trails. So, I can really only ride on weekends, and some urban stuff at nights.
I live at sea level. A lot of the good nearby trails are up near Tahoe at 7,000 to 9,000 feet. To be able to climb on those trails (especially when summer comes around and I have more time) up at that high of altitude, I need to be in decent shape.
I have a 20 mile commute from my apartment to work that I drive five days a week. If I got a road bike and was riding that instead, that would be up to 200 miles a week on the road bike. That should keep me in pretty good shape for weekend rides and next season, right?
Anyone else train on a road bike?