first, the bad news. I had this and couldn't run for fifteen years.
second, the good news. a simple set of quad strengthening exercises will cure it. when I finally talked to a trainer who knew what she was talking about, I got the exercises and they worked. In your case, I'm betting that the muscles haven't atrophied to the point where it will take much time to fix. You should be running very well again within a month. If your knee can hack it now, you can go right to the leg weight where you sit and push a weight up with your shin from 90 deg up close to horizontal. My knee was too weak to do that even with 10 lbs so it made my knee unbendable for a month. So, you can start with ankle weights. Google this or:
on back, one knee up (foot on floor), right leg straight out, bring rt knee back to chest and forward straight out again to the floor, x20. do the other leg.
on back, one knee up (foot on floor), lift straightened right leg up to left knee level and down slowly, x20. do the other leg.
on right side, with left leg bent and on the floor, lift right leg up about 18" and down slowly, x20.
still on right side, with right leg bent and right foot on floor, lift left leg up to right knee level and down, x20.
repeat on the left side.
sitting up on floor with legs straight out, tense quads and hold for 3 to 5 seconds. x20
lying on stomach with one leg straight on floor, bend other leg 90 deg and lift straight up and down. x20. do other leg.
sitting in a chair high enough so that your feet aren't touching the floor, slowly lift one foot up to horizontal and down. x20. do other leg.
standing, pull foot behind you to rear and maintain 30 seconds. do other foot.
standing, pull foot back toward rear while using your quads to try to push the foot forward at the same time. maintain 20 seconds. do other foot.
do squats with no weight.
All these can be done every other day (even two sets if easy and the weight in the ankle weights can be increased: you can move up to weight machines when your quads are strong enough).
The last thing should not be done more than once or twice per week:
thrumming: breaks up scar tissue by pushing hard and rubbing with your thumb or forefinger on the tendon that is just above the next bone down from the patella.
no jokes, all tried and true. it works.
If you're getting tight hamstrings from these exercises, you can counter them by doing some backwards walking or even jogging (just a few yards per day after a run), or (less effective) with some Russian hamstring curls (basically, like you are praying to Mecca, on your knees, hands across stomach, bow down slowly until your head touches the floor and rise back up, a few times).