I think the type of exercise here is significant. Exercise in general will make you feel healthier and improve your self-esteem, so that's good, but I think there is something else at play with cycling, which requires intensive concentration and engagement with your environment. You are moving at speed and exposed to the elements, so you have to be focused on what you are doing and what is happening around you. This doesn't leave a lot of room for DP thoughts, so I imagine that if you did it for long periods of time, like a biking holiday, it could have a real benefit of drawing you out of yourself and retraining your attention.
So the interesting question is: How much of the positive effect is due purely to the physical exercise and how much is due to the mental engagement with a real world environment the activity requires? I expect they both play a part, so hopefully it may not be necessary to do these things to the extreme - interestingly there's an old technique monks would use to teach students clarity which was to make them run up a mountain until they dropped! Of course it's not possible for everyone to cycle five hours a day, even gently, but maybe there are some principles at work here that people can adapt for themselves. (Always consult your doctor before beginning any new exercise regime, yadda yadda yadda.)