Mountain biking can be an excellent way to get fit and stay fit, without incurring the types of injuries that happen to runners (and hardcore mountain bikers who periodically fall off their bikes). It’s possible to have a lot of fun and get an excellent hard workout without heading into the dodgy hilly tracks that the younger mountain like so much.
Mountain biking trails are great fun, and the climb up the hill can be a lot of hard work but this hard work is generally very peaky, and the effort is not sustained but is almost always in short hard bursts. The trails come with a certain amount of risk if they are anything other than easy grade, simply because they present so many opportunities to fall off.
One aspect of running that runners love is the opportunity to keep up a sustained and steady work rate for 30 minutes or an hour or however fit the runner is. Obviously this will not apply to hilly off road tracks, but a reasonably level off-road trail will allow the runner to keep their heart rate in the mid to high 140s for most of their run.
This type of work rate can be duplicated on road bike riding on the flat, but the actual off-road experience can be much more exhilarating on a mountain bike riding on reasonably level off-road trails. These will have a mixture of smooth and rough and undulating and straight and curved, and the riding will be twice as fast as running and that much more exhilarating. Riding also put much less stress and strain on the leg muscles and tendons and joints, and so is much more sustainable over an hour or more then the equivalent run.
For the keen rider an iPhone mounted on their handlebars showing the RunKeeper app or similar will add a high degree of motivation and challenge, and many riders find that riding to an average speed is even better and more motivating than racing. The narrow parts of the track really need to be clear of walkers and their dogs to be safe, and so an early morning start is a very good idea. The challenge for the rider will be to ride at maximum speed without “blowing up”, and this can be a lot of fun when the track has rough parts that need concentration and some expertise and care. Because the modern mountain bike has such good gears, it’s possible to sustain a very constant in high work rate for the duration of a ride, and the great part is that all this is recorded for posterity on RunKeeper.
Another good feature about mountain biking for fitness is that it does not strain the leg and ankle joints and tendons nearly as much as running does, and so the bike rider is still able to walk around normally after a really hard bike ride instead of hobbling around.