Mountain biking is often promoted as an exciting, exhilarating form of cycling that is fast and full of descents. However, for the most part it is also full of climbs, slippery ground, mud and obstacles. To a large part the descents are more difficult than the climbs. If the rider is not ready for the whole package, they very quickly become disheartened. The other thing that beginners sometimes under estimate is the distances travelled and the length of time that riders might stay out. A normal spin might last over 3 hours. So if you are someone who can't last 20 minutes in a spinning class, then a 3 hour cycle is definitely too much. Try to team up with other people who have the same level of fitness and build on your ability from there.