Hi spambot, I can definitely sympathize. Knee pain is no fun, particularly when you don't know what's causing it.
From what you've told me, I suspect
it's tension in your IT band. Your IT band is a very long band of fibrous connective tissue attached to a very small muscle (TFL) just below your hip and the lateral side of your tibia. It crosses your knee on the front, just to the lateral side of the patella.
It's a somewhat common injury for people who are ramping up their activity level and is particularly common for people who over-pronate since this misaligns your lower leg and places additional stress on the ITB. The typical presentation involves pain in the band itself above the kneecap, but I've seen quite a few cases of pain where you're describing where it was actually the bursa in the joint that was being inflamed by the ITB pressing on it.If I'm right
, it isn't particularly serious, but it isn't fun either.
RICE is usually helpful. You'll want to ice/compress the joint itself since that's where the inflamation is. I'd also recommend massaging the TFL muscle which is on the lateral side of your thigh from the crest of your hip to about a third of the way to your knee (it varies person to person). Massaging it with a foam roller (or a tennis ball if you're a sadist) is a great way to do this. Definitely extend your warm up and cooldown as well and stretch it more.
The stretch I normally recommend is this one because it's easy to do and hard to get wrong.
Lie on your back (resist the urge to try this one sitting, the muscle is partially pinned by you sitting on it) and cross the afflicted leg so that its ankle is on the good leg's knee. Grab the good leg's thigh and pull it towards you. You should feel the stretch on the outside of your bad leg's thigh just below the hip.
Hold the stretch 30 seconds, relax a minute, then stretch it again for 30 seconds. Do this every day, though don't do it immediately before running. Long static stretches immediately prior to a workout do more harm than good. Studies have repeatedly shown that the short-term benefits of stretching go away in 3-8 minutes depending on the quality of the stretch, whereas the effects of restricting the blood flow last significantly longer and increase the chance of injury. This applies to all stretches.
Please take all of this for what it is, advice from a stranger on the internet. If symptoms do not improve, get worse, or you want to do the smart thing get a referral for an orthopedist who can actually examine you in person and get a much better sense of what's going on and how to treat it. I'm a certified trainer, but without being able to see you in person... well, for legal reasons none of this should be considered professional advice and I accept no liability for how you handle this whether or not you listen to anything I've said.
On an anecdotal note... don't accept your age as a limiting factor. There are hundred year old marathon runners out there. What you're dealing with is the growing pains associated with getting in shape. Once you're in shape you'll be able to maintain it and be healthy long term. I know so many people (family included) who give up at the first sign of pain and write off their health as a loss.
It's heart breaking to watch because it's almost never true.
You need to properly habilitate your body. If you do, short of major trauma or a condition such as rheumatoid arthritis you don't ever have to give up on your bones, joints, or muscles. I'm about your age. A year ago I fell 20 feet and landed on one leg destroying my knee in the process. After no surgery but some long and very difficult rehab I'm at 100%. Full range of motion, full strength, no pain.
Never give up.