I used to go to the gym but I found the driving time sapped my motivation, and then I wouldn't go very often, and then I cancelled my membership. Recently I have been getting back into resistance workouts but have been doing them at home. There is a lot of good information online on good workouts you can do at home with just your body weight, and maybe a few things like a medicine ball, swiss ball, and some free weights. Without a lot of expense you can get a variety of decent workouts without going to the gym.
I love doing this because I can do my whole workout at home in about the same amount of time it used to take me to travel to the gym! This makes it a lot more sustainable for me, because there's not a whole load of effort traveling etc etc. I like to work out Mon, Wed, and Fri, and I have a few different workouts that I rotate between to keep it interesting and to give my body some different challenges week to week.
As for running.... well I ran a few years ago on concrete without properly researching about what needs to be done to protect your knees. I don't think my shoes were ideal, I was only running on hard surfaces, I was heavy, and I probably wasn't stretching and exercising my legs in the optimum way to protect my knees, and I ended up with very sore knees after a couple of months. I stopped running, but have had trouble with my knees ever since because I have been too lazy to do my exercises. However since I started doing my exercises routines at home, I have been building the strength up in my legs a lot, and my knees are no longer sore. MUCH better than they've ever been since I stopped running, and nearly as good as they were before I ran