Fast and sustained mountain bike riding can be a great and safe way to get fit and stay fit. A modern lightweight mountain bike is ideal for this, particularly over reasonably level tracks such as are accessible from most New Zealand cities and towns.
The modern mountain bike allows a rider to maintain a very constant work rate, simply because it is so easy to change gears. If the rider is using an iPhone mounted on their handlebars then they can be monitoring their average speed and heart rate to make certain that their work rate is sustained and constant. This is very similar to what gym uses are accustomed to on the stationary bikes, but it is vastly more fun and exhilarating. Off-road tracks like the Hutt River trail for example provide a wonderful riding environment, with wonderful scenery and bush and riding challenges that means that the riders mind never wanders. The rider may not notice much of the scenery if they are concentrating heavily on the track, but they are still out there doing it and the enjoyment is always immense.
There are a number of advantages of this type of writing versus what one can do in a gym,and these include the psychological advantage of being in The Great Outdoors, the cooling advantage of riding fast, the high level of concentration to keep the speed up and the work rate constant and the fact that the entire exercise is free and very satisfying.
The rides are best done early in the morning in order to avoid walkers and their dogs, and this means that the rider can go hard out and take some risks that they would normally avoid if there were people around. Sustaining a high work rate is fairly easy over the sealed sections of the track, and also it is easy over rough sections although the rider may find their heart rate increasing significantly as they work hard through deep gravel or while negotiating around potholes.
If the trip is out and back then one leg of the trip could be into the wind and the other leg with a tail wind, and if the return trip is the tailwind then this can be truly exhilarating and at least 10 kilometres per hour faster. Paradoxically the rider may find their heart rate increases on the return trip because of the exhilaration of the high speed ride and the potential to ride even faster. A good test for anybody is to ride out for 11 km exactly and then turn around and return, and make the whole trip under one hour. 22 kilometres an hour or higher is a very good average speed for an out and back trip over partially rough terrain.