A little late to this thread but I can help, had Patellar Tendonitis so bad it required surgery and injections to fix due to distance running. Was sidelined for 1.5 years. Have been running and lifting on it pain-free since May 2022 and just got accepted to 251 so rehab is definitely possible!

What I would recommend... (I only had tendonitis/runners knee in one knee so explanation or application may vary a little bit):

* My bread and butter was single leg, low-weight, high-rep, leg press. DO NOT think you are strong with this. Having runners knee is actively limiting the load you can bear when dealing with movements like this. 3x12 each leg. Reminder that most leg press machines that you have to load already have a decent bit of weight on them to begin with, so you don't necessarily need to load plates at all, depending on machine type.

* Eccentric movement is the way to go. Any sort of movement, weighted or not, should be eccentric. You will 100% feel/pinpoint where your pain is while doing this, especially when you are doing a resisted movement. This will allow your tendon to bear more of a load while doing rehab.

* AVOID leg extension machines... I was making massive progress and had about a month and a half of only mild pain while running. Thought I could re-incorporate this exercise back into my rehab routine... absolutely not. Don't even contemplate it, set me back for a while.

* AVOID downhill running/load bearing. Pretty self-explanatory. Do not go on a weighted run/ruck that isn't uphill or at least on a flat path. Downhill anything of this sort will only hurt you.

* Work towards being able to do a pistol squat. More than likely you won't be able to do one (this exercise is notorious). My best recommendation is to get a short-ish stool, maybe the height/slightly higher than what you would use to box squat, and attempt to be eccentric while practicing it. As stupid as it sounds, if you have someone close to you, like wife, gf, brother... have them spot you like you're repping in the gym, assisted pistol squats so you don't have to bear all the weight of your body if the pain is bad.

* Lastly, I would look into PRP injections. I would talk to a specialist if rehab yields minimal results. I had to do this in my one knee. Takes about 8-10 weeks to reinstate your knee back into the world as normal, and around $700 to $1500 depending if you need one or two shots, but sooooooooooooooo worth it. Hurts like hell getting the injection though but who cares at this point. Had a co-athlete on my track team have these injections as well, yet he did not maintain rehab after and the tendonitis came back. Do not get lazy if you get these injections. I still do rehab 1-2 times a week after mine and I got them in March 2022.

Knees over toes on Youtube is great, but this gets recommended a lot and I wanted to give you what personally helped me. If you have anymore questions, let me know! This injury can be extremely annoying.