My eldest daughter represented Australia in the womens 20K walk in Berlin. Sadly she got DQ'd just beyond 10K. This probably had more to do with the fact that she'd had a child 4 months earlier, so her training had been somewhat compromised. The speed was still there but the technique was rather rusty. Stiil gave me time to get very teary and proud though.
As an event its no dafter than throwing the stick or jumping into the sandpit. Perhaps you'd like to try telling the German guy who won the shot how silly he looks, I'm sure he'd be happy to discuss it with you.
Injuries tend to be hip and back related. Knee injuries are less common than with distance runners.
Overall a walker is likely to remain injury free for longer than a runner. The seriously competitive window is longer too. There are a lot of world class walkers who go on into their late 30's.