I've never run a marathon, and I'm not sure I'd like to. My father ran marathons and did triathalons when I was growing up, and it was always really exciting. But now he says that was the folly of youth, and he thinks marathons are too rough on the body. I'm not sure, but I myself would probably not do it because I had JRA (which has been in remission about a year), and so my joints aren't really up to snuff, although I ran whenever I was well enough growing up. The longest I've raced is 10 miles, which is a good distance. Actually I think my favorite distance to race might be 8k or 5 miles. But like you say, I may change my mind someday
I've tried to cut out diet soda, but it's hard. As a woman, I worry about my bones and calcium intake, and soda is just about the worst thing I could take in on that front. I don't really think multivitamins are so healthy, actually. It seems we should get our vitamins in our food, and taking too much might be bad. I know they periodically come out with studies showing that high vitamin intake isn't so great, like recent data concerning vitamin E; then again, it's the fat-soluble vitamins we'd worry about, I guess. I do take a multivitamin currently, but that's only because my husband and I want to have a baby before I go to med school, so I have to make sure I get enough of everything.
Oh, and also my mother sent me this book recently called "Dr. Ann's 10-Step Diet", which talks about diet soda. Now, I'm definitely not a person to follow a fad diet, but neither is my mother, and apparently she got turned on to this book when she met the woman (who graduated top in her class from MUSC in Charleston, where my parents live). Anyway, this Dr. Ann woman mentions that some people have an insulin response to just the sweet taste of diet soda, without even having the sugar, and that she had one patient who lost a bunch of weight just cutting out the diet soda. I guess that's plausable. She says it's only some people, though.