frequent endurance exercise is not agreeable as I hear it. I have heard of long term studies being done on endurance atheletes and serious illness, an example would be that it has been suggested that Lance Armstrongs cancer could have been bought on/exasperated by overtraining in endurance events ( I am not sure if he agress with this or not) and the same goes for numerous other atheletes.
I am not sure myself but i do have a sort of first hand experience, when I was younger I would simply never stop whether it was structured exercise or simple play time, i was accused of being hyper active, I would run 2/3 miles every morning, play sports every day - i did this for years, my doctor advised i was the most well put together person he had ever seen, until one day i got bone cancer for no reason that anyone could figure. I feel it may have been a combination of many things but I cant imagine the constant pounding of running and constant exercise was any help to it what so ever.
Overtraining in anything can be very bad.