Great. That 4 miler. It's really good to have at least half a mile at a gentler pace than going 100%! And if you're ever doing a serious attempt at a 5K time trial/park run, it will make quite a difference to jog a mile first, then do some hundred+ metre runs approaching race-pace to get all those physiological systems turning over.
There's obviously some simplifications in there.... but hopefully helpful to get you to the next stage, and get you reading more about the subject.
Good luck with those knees. Get good shoes. Run on softer surfaces when possible... and work out if tennis is damaging your knees. If so, think about taking some tough decisions. If they're getting knackered already... a lot of your life is spent over the age of 40 and you don't want to be restricted.
Where I said do tempo runs one week and intervals the next, I would really focus more on tempo type runs 2 weeks in every 3.
Interval training is done at faster speeds. This definitely has its place and is good for you, but given where you are, I would have a bias towards doing aerobic and tempo runs.
Also, to be clear, when you run your 5K race (which I'm assuming is your short/mid term focus), most of your energy comes from the aerobic system... so the more efficient you can make that system, the better. But you do dip into those anaerobic systems, with the nasty biproducts... so you do benefit from that speed work.