Ed, I'm probably not the best to answer this because I haven't tried it myself. Saying that, I did watch the U of T Parkour club practice every week, and talked to quite a few of the members, because they practiced at my favorite (warm weather) study spot.
I have to say that these guys are extremely well conditioned. They start practice off with a twenty minute run, take a water break, then do sprints, pushups, and squats. After that, most of their conditioning is done by using the space around them as an apparatus. After the new guys learn the basics of break falling, they begin to work on jumping. Initially, they just jump from a squat position across the grass, then roll and stand. After a little of that, they find a path that is lower than the surrounding ground, and jump from side to side, landing in a breakfall. They also hop up stairs, taking them two at a time. There is a large statue in the park, and some of the more advanced guys use that in various ways, for pullups, as well as balancing on. I saw a few use a tree stump to do a headstand on, and then do raises in that position. They also practice railing jumping, initially not to high up, but as they progress they go higher and higher up the stairs. It was amazing to see people who had little strength at the beginning, being able to bound up a stair rail in five or six leaps, hoping to either side, in a matter of a month.
The training leader seemed to have things fairly well organized, and it does seem that they follow a pretty tight protocol, taking someone from beginner to advanced.
Aside from the training sessions, they meet in the park, warm up, and then go for a free run. These guys put ninja's to shame.
As for injuries, apparently they are pretty common. Mostly twisted ankles, and sprained wrists--but occasionally concussions, and cuts that need stitches.
Here are a few videos that show the potential dangers of this sport very clearly. They highlight the need to actually learn from someone who knows what they are doing. Warning, some of this stuff hurts just to watch...http://ca.youtube.com/watch?v=vME2BBzxK38http://ca.youtube.com/watch?v=Ezi04hhcYm4http://ca.youtube.com/watch?v=P4wOByXdM1M