I've been running for years now, started a few years before I had my surgery. They only differences between now and then are I'm faster, have less weight on my knees, have excess skin that makes for funny in motion pictures, a am not out of breathe as quick. When I first started I downloaded a free C25K (couch to 5k app). It was a good way to start and I adjusted as I needed to as I was not in the best shape. Fast forward to today, I run 5k races a few times a month. They are my accountability to keep exercising; my goal. This year alone I have completed 2 10k and have 2 more 10ks planned as well as a half a half marathon.
My advice, take it slow. Don;t push yourself. Interval run/walk methods can be your friend when you're starting out. Don't get discouraged if you are unable to run specific distances without stopping. You also don't need a PR )personal record) each time. For instance, last weekend I went into my 10k don;t feeling that great. My goal was to just finish, and preferably under 1:20:00 min. I did just that. My time was actually only about 30 seconds slower than my May 10k time. Imagine if the course had been flatter, I might have actually beat it.
Good luck.