There's a whole loada stuff to do. I do a spot of free running myself, although it's more like climbing up random stuff rather than running, but sometimes we have to run away from stuff too, and it helps to have some kind of knowledge of your limitations, like what jumps you can and can't make, how to land when jumping down, or how high you can jump, and how to make it over a high fence, or up to a window....anwywhooo...

I also do boxing, and for an older guy, I think it can be good. If you're not properly toned with weights and loads of resistance training and stuff, and if you're not the quickest/the lightest on your feet, then I wouldn't suggested something like kick-boxing, because you need high kicks, and kick and thai boxing are pretty extreme, whereas normal boxing is just hits to the head, and if you're just training, then they'll give you helmets mainly and say no face shots.

It's really not all about punching to the ace, and I've done a lot of running, and football (soccer), and boxing is by far the hardest in terms of cardio, because you're always moving, and if you're not, they'll take advantage and beat you silly. Obviously you can get guys to go easy on you too, and they will if the know you're new anyway (unless it's a messed up gym). Also, there's much less actual fighting, and ,much more practice on techniques and getting them right. You can practice this at home too, just buying a punch bag or something, or having a nice bit of hollow wall you don't care about damaging, and other little things, like you can just stand in front of a mirror for about 5 minutes at home, get like 2 Kg weights in each hand and just box, use the things you've learnt about technique to make sure you're doing it right, and the weights make your punches faster. At a decent gym they'll give you loads of different little exercises you can try out to keep yourself in shape, and at the end of the day, you're learning how to beat the crap out of people, which is useful...

Paintballing is fun, but you can try airsoft or something like that if you don't want to spend as much money.

A sport like that raquetball thing sounds good, although badminton is a very showy sport. If you're decent at badminton, you'll know some very impressive party pieces, and I knew some guy that did it, he was awesome, and ended up playing for England at youth level.

Just on running, weights and resistance training and stuff, I didn't have huge joy doing these things, but I did enjoy a couple of aspects. Firstly, it's nice to feel like you're doing something that will improve your fitness. Keeping fit helps you in pretty much all aspects of life, there's not really any downside, and this is good to know. Secondly, I preferred seeing the results afterwards than the exercises themselves. It's great you want to turn a new leaf, and especially for your kids, but the satisfaction of going for a run or something like that should come out of knowing that you're reaching that goal. yes, in an ideal world there would be something interesting, cheap, easy and accessible that can keep you properly fit, but there isn't. Just try stuff and see what you can bare doing.

Also, the physical challenge is a nice challenge to have, pushing yourself so you can go further, and trying to get better. I'm just a standard brawling boxer, but one of my good friends is more of a finesse fighter, he does proper martial arts, and has done for about 10 years now, and he said that one of the hardest, and most useful skills for his martial arts is breathing. Controlling your breathing is extremely important and complicated, mainly because, as you said, it's hard to focus utterly on something that seems so easy and basic, and to know what you need to do with it, there's much more to it than panting and putting your hands on your knees...don't underestimate breathing man . And another thing. It can be pretty nice to just run to keep your mind off things.

P.S. I find a pair of headphones can turn a dreary run into that part in Rocky 2 where he's running .