I clearly upped my mileage too quickly from zero back to something close to normal too quickly and have knee bursitis/tendonitis, and a walking stick for the past week!
I've got that at the moment.
I first got it when I did a few 24- 26hr single speed MTB rides over hilly terrain. It was because like you I'd upped the intensity of my rides. Pushing a large gear for a long period of time.
I recovered and everything was fine for a month or two then I sprained / factured my ankle. Because my ankle isn't strong enough to keep my knee straight while i'm walking /cycling I keep aggrevating the knee joints and getting tendonitis in the knees again.
I've found stretching and ultrasound to be highly beneficial.
(Its worth a go but if anything hurts or if this causes further aggrevation then stop immediately.)
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Lie on the floor with your legs straight.
Straighten the knee, point your foot upwards then lift the foot 20cm from the floor and hold for 10 seconds.
Straighten the knee, point your foot to the right and then lift and hold for 10 seconds.
Straighten the knee, point your foot to the left and then lift and hold for 10 seconds.
+ Quad stretch
Stand up and touch your bum with the heel of you foot, hold for 10 seconds.