I guess it was just my luck to lose a nail (yes, that's what that is) during the marathon training. The damage was done during the Long Beach race but did not finish its course for months. In spite of how bad this may look, it was not painful, and it was replaced by a healthier one.
How do you know that you've damaged a toenail? Well, if pain is a good indication. It might throb on its own or feel bruised to the touch. It may also change color.
I've had these symptoms without losing the nail. So far, the pictured specimen is the only one I have so far. I never thought it would fall off, but months after the injury, I noticed it turned white (it was formerly bruise-colored) and very loose. It pretty much came off painlessly by itself.
On the other hand, I had a worse injury as a result of a 16-miler and decided to intervene because it was a larger nail and looked/felt really bad. I gingerly released some liquid building up from underneath (had to repeat over days because it kept refilling). I think this kept the nail from falling off too soon. Four months later, the nail is still there, growing out. I didn't want to have a "naked" toe, so I guess this was the best thing I could do.
Some people actually puncture the nail directly to release the fluid, which is probably what I should have done. I don't think I could bring myself to do that. They heat up paperclips so that it just burns through. If you ever decide to intervene on a bruised toenail, use some sort of disinfectant, lest it end up hurting more than if you'd left it alone. These are one of those injuries that heal on their own in time.