Paul, welcome back to running.
First off, I’d say don’t break the bank for a gym membership if you don’t have a real desire to. There’s nothing wrong with running outside. If you dress right, you’ll be fine. Personally, I have access to treadmills for free pretty much whenever I want and I haven’t been on a treadmill for I believe over a year.
Here’s my usual suggestion for starting. Go out and run as slow as you can. See how long you can go. If you make it for 20 minutes or more, that’s a great start. Keep running for 20 minutes or a bit less than however you made it on a regular basis, at least 4 times a week. If you can’t make it 20 minutes, then mix in walking breaks to start. Your goal should be to get in at least 20 minutes of movement again at least 4 days a week so shorten up the distance you could run a bit, then walk for a bit, then run again. For example, if you could run for 7-8 minutes, maybe you start by running for 5 minutes then walk a minute, then run for another 5 minutes and continue until you got in 20 minutes or so. As you get more fit and this becomes easier, you can increase the duration of the running portions and/or decrease the duration of the walking portions until you are running the whole way. As for how quickly to build, do so when your body feels ready. Remember, these runs should not knock you completely out. Once running the whole way, again, let your body guide you as you build the duration.
My suggestion is to try to go without a knee brace. One thing about wearing braces of any kind. If you wear them for too long, you lose strength in the body part that you are wearing them on and you end up becoming dependent on them. If at all possible, you’re better off trying to bild up the strength of your knee. If something doesn’t seem right, get medical attention for it and find and treat the root cause instead of trying to treat the symptoms.