Check out this article at slowtwitch.com. It's a phenomenal article on how marathon training tears you down.
I always say: "Half marathon runners are always hotter than marathon runners. 10k runners are hotter than half marathon runners. And 5k runners are hotter than 10k runners." This article, while it approaches it from an anti-aging perspective, explains why longer training makes you sick, injured, and less attractive.
It's even more interesting when you note that the author is a former sub-2:20 marathoner and a 4th place finisher in the Hawaiian Ironman.
Training is no guarantee of health
Let’s get one thing straight right off the bat: Endurance training is antithetical to anti-aging. So it amazes me when guys in their 40s and 50s who are training for a marathon or Ironman suggest that doing so will keep them young. It won’t. You may feel like a stud now with your shaved legs and your magic marker biceps tattoos, but endurance training speeds up the aging process almost as fast as watching TV, drinking sodas and eating potato chips. Actually, in some cases, it speeds it up even faster.
Read the rest here: http://www.slowtwitch.com/mainheadings/features/health_doping_slowtwitch2.html
I also thought this was pretty good – from Mark's official bio on his blog:
"In fact, the running was going so well after college that I decided to forgo medical school for a few years (it’s at 31 years now) and concentrate on a running career. I trained seriously as a marathoner for another five years, racking up well over 100 miles each week in training. The effort culminated in a top 5 finish in the 1980 US National Marathon Championships and a qualifying spot for the 1980 US Olympic Trials. Unfortunately, by then the inhuman amount of training and weekly racing was taking its toll and I found myself constantly sick or injured. (Note to self: too much exercise is not a good thing). In fact, in my last year of competition, as a world class, extremely “fit” athlete, I experienced eight upper respiratory infections! Clearly I was ruining my immune system and my joints doing too much exercise. That’s when I started exploring nutrition and supplementation as a way to enhance my performance and to support my damaged body and bolster my immune system."
Check out his articles on all kinds of cool stuff about fitness, looking athletic, anti-aging, and health – http://www.marksdailyapple.com/
For another look (possibly a more balanced look) at Strength Training vs. Cardio, check out this article from this month's issue of Women's Health:
A WH Fitness Face Off
In one corner: Dumbbells. In the other: A jump rope. The ref: WH, slicing and dicing the research to determine whether strength or cardio rules.
To resolve the strength vs. cardio conundrum, we culled research and chatted up experts to find out how each would fare in a head-to-head matchup (don't worry, nobody's going to bite anyone's ear off). Whether you want to get buff, torch calories, or run your fastest mile ever, we've decoded which discipline you should devote your sweat to — and created a workout that's perfectly proportioned to give you all the benefits. Now, let's get ready to rumble…