From a runner/weight training perspective. You are absolutely right to worry about joints, here is my suggestion from a lot of training experience. If you want to do squats, get to the point where you can do 150 correct squats (that is all the way down and all the way back up continuously, as in don't stop to rest, before you put any armor on to do the same exercise. Same for pushups or situps. Go all the way down to where your chest touches the ground, do not lay on the ground, always keep the elbows under pressure, before coming all the way back up and then continue, no breaks in between. Shoot for a goal that is excessive before you add weight because what you need to do is build up the joint muscles to be strong enough to take any additional weight. For sit ups, do them properly, only allow your shoulder muscles to tap the ground, do not rest there, and immediately back all the way up so that your chest touches your knees and back down without swinging your arms or pulling on your neck. If you are not yet to a point you can do these exercises properly without armor, then don't add armor. When you do add armor, start slow. Aim for 1 correct of each and work your way up slowly. Muscle is not easy to build and takes time, it is supposed to.
As for running in armor, don't even try it unless you are very well conditioned to run without it. As someone who was a runner, and who got a stress fracture from it, and who had to run on said stress fracture, save yourself the pain. Start by walking 5 miles without armor at an increased elevation, then add small amounts of weight once you can walk that distance at a good pace. Always begin at the beginning.
Above all, to prevent hernias and other such injuries, remember one very important rule. BREATH!!!!! Pushing excessively while holding your breath causes hernias. If you do a pushup, breathe through the entire thing, if you are doing a situp, same thing applies, if you are trying to lift a stack of weight, breathe! I can't say it enough, and it is hard, probably harder than the exercise itself, but it is of utmost import.
/end 2 cents
