like @Bayrunner, i'm not a certified fitness expert or whatsoever, i'll just give my opinion based on experience and what i've learned so far...
kung ang tanong is running the marathon itself, technically it's unhealthy in a sense na you're breaking down your body, thus, tilting the metabolic balance in favor of catabolism(breaking down of tissues).
given that, i think there is a certain limit of full mary's that one can participate in a given period of time.. say, once a quarter?(im not sure, havent had a FM yet)
however, the healthy part is actually in the preparation part of a marathon.
you discipline yourself in training, you exercise regularly to strengthen your body, you feed yourself with foods of high quality nutrition, etc...
this, i think outweighs the breakng down done by the actual marathon run. of corse, proper nutrition and stretching exercise is also required after the run itself.
another thing is that we shouldnt confuse marathon with running. generally, runnng is healthy... running a marathon requires optimum health level to be able to endure it. running is a lifestyle, and for some, a hobby. a marathon is just one event of this lifestyle and i guess, should not be a hobby :))