val_owI
I think it's a common misconception that you have to train like a gymnast and be heavily monitored when training parkour. When I first got into it I was the most unfit person ever. You don't go into parkour being fit, but rather get fit from doing parkour. It's just another form of exercise. And so the more you do it, the better you'll get at it because you'll be getting stronger too.
The training itself, you've just got to use some common sense and know your limits. It's good to push yourself, but you need to know when it's good to do so and when it isn't. See if you can find a local parkour group if you can.
That's really the greatest part about functional training, you actually get better by being functional. I probably really forgot to stress this enough, but just doing something will make you better at it in your mind and body, which is really cool. Some of the most fit people I've ever met hardly did anything fancy to train one area, just hard workers. My dad always used to tell me when I was younger that sports and lifting and such were basically just a game for people in the older days cause they were already in shape from all the manual labor they had to do- I'm not sure if this was everywhere but Chicago was considered one of the industrial capitals of the world back in the day. For instance, I don't want to brag on his behalf or anything, my dad was pretty extremely successful (setting records and such and in the hall of fame at his school) at sports when the only real extra work he would do was running which wasn't too hard for him anyway since he would work very hard everyday at the business he was a part of. Also, at least in my sport, farmers and other hard working kids often end up very good at the sport that I do.