Re:Lack of leg training = bad joints?
Good functional exercises performed well will help a lot. A side effect of training is the thickening and strengthening of connective tissue, as well as strengthening the muscles surrounding the joints.
You have to do it right though, avoid leg curl and leg extension machines (or if you do use them, stay light and rep out). Concentrate your efforts on squats (free weight) and deadlifts. These are compound exercises and will help with your overall development more than any of your upper body stuff, and are also functional (ie, those movement which are natural daily movements throughout life) so should help a lot.
Obviously, piling on massive weights too soon could have a risk element so get the form nailed and build up slowly. You should have been training the lower body and back from the start, these big movements are the key to getting the fitness and physique you want.
I doubt there's a pro sportsperson that doesn't squat, even lots of distance runners, triathletes etc squat these days. It is the dogs of all the exercises and most people, if they could only do 1 exercise, would choose the squat.