When I was young I would occasionally have issues with patellar tendinitis. What worked for me was a short lay-off with a bit of aspirin and lots of stretching, particularly the quads. When I get back on the bike I follow a religious adherence to keeping my cadence above 95 and do lots and lots of leg extensions in the weight room (with a few curls to balance things out). I do the extensions one leg at a time with very low weights. Fifteen reps with each leg, ten of these with each leg is a set followed by rolling over for curls. No breaks and I'll go for one to two hours adding five pounds per set until I reach fatigue and then subtracting five pounds per set.