I’ve heard that mountain bike athletes train on road bikes to build their fitness – does this mean that sticking to mountain biking only will get me less fit?
They ride on the road, but not necessarily on a road bike. I coached a young rider to a national championship win once and he never owned a road bike! Riding on the road allows you to control your pace better, which is especially important for the longer aerobic rides. One of the problems with riding off-road is that the pace invariably becomes intermittent as you’re constantly putting the power on and off (surging – then freewheeling) to get around obstacles etc. To build a solid aerobic base, you have got to spend some extended periods of time on the bike riding at a constant pace. This can then be supplemented with off-road riding to build anaerobic power and technical skill.