I don't think I could do justice to the excellent responses to your question. But please allow me to add these thoughts. I have known excellent "martial artists" that have had their clocked cleaned on the street (it's always kinda funny pulling up on a call with a guy on his back, bleeding from the nose, wearing "I attend the worlds greatest martial arts school" shirt and standing over him is a drunk longshoreman). And I have known individuals that have never been in fights in their entire lives scare the crap out of attackers, without ever throwing a blow. There are no secrets.
Training in anything within you capabilities is better than nothing. You can take a knife just about anywhere and not have to worry about ammo or airport security. But the bottom line is, its better to win the battle without firing a round (my Sun Tzu paraphrase). Train for awareness, its cheap, you don't have to bow alot or learn to count in Japanese.
ps. I can feel for you on the back issue, a couple of on duty traffic accidents and the weight of gray hair has taken its toal.