I attempted to do 21.2km (slightly more than a half marathon). I failed.
At 1 hour and 47 minutes -- that's almost an hour longer than I've ever ran before -- my muscles stopped. I'd been getting gradually slower for the past half hour; at the time I wasn't really going faster than a fast walk. I wasn't panting at all, nor was my heart rate fast. My muscles just stopped responding.
Actually, I had some difficulty earlier -- shortly after beginning my second lap, my right knee started to hurt. I considered turning back, but I stupidly decided to keep on going. Don't do this at home, boys and girls. When you're panting and you generally hurt, sure, keep on running. When you feel pain in your chest... well, you should probably stop running, but I keep on going. When you start to lose circulation to your fingers... again, that's probably a really bad sign and you should slow down or stop, but I keep on going.
However, when your joints start to hurt, stop. I mean, even I will stop (in the future) when that happens. And we've already established that I take a lot of stupid risks while running.
Anyway, when my right knee was hurting, I limped along by putting more weight on my left leg. After about 2km, my right knee felt ok (or just stopped complaining), so I started running normally for a while. But then I noticed that I was slowing down, and couldn't speed up again. I wasn't out of breath, and I wasn't in pain... I just couldn't get my muscles to move faster. After a while of that, I tripped slightly and started walking, and couldn't start running again. I couldn't even walk normally... it took me half an hour to reach the car, and a pair of elderly ladies passed me. I was going really slowly.
Anyway, it was an interesting experience. I think I definitely qualify as "injured" -- I couldn't get into bed without lifting my right leg with my hands -- and it certainly qualifies as "self-inflicted" and "caused by stupidity". But at the same time, the only way to learn about this kind of thing is to experience it once or twice. I would have never paid attention if anybody warned me about this before today... actually, come to think of it, I think a few of my friends did warn me.
And I certainly chose the right time to do it -- I'm living with my parents, with no immediate deadlines. I have a few days until I play in orchestra or do any music teaching, so my daily life is relatively untroubled by this injury. It'd be terrible if this happened at UVic -- I wouldn't be able to bike or walk to university. Now, I'm sure that my landlord would have given me a car ride... but still, I'd rather not be wandering around the university using the hand rails to pull myself up stairs.
Yes, definitely a valuable experience. And hey, how often do you get to say "yeah, I failed... I only ran for 17km without stopping"? :)
PS: running a marathon is definitely a "when", not an "if".