In reply to The Potato:
I 50% severed my patella tendon while running, a very sudden injury and really quite painful. It basically stopped me from running seriously. I'm not sure if there was pain leading up to it, at the time I was about 19/20 doing loads of different sports and running a lot, so I was simultaneously always tired and a bit in pain, but also totally fine because I was young. I'm trying (not very successfully because I'm lazy) to get back into running now approx. 8 years later. I'm sure with better rehab I could have got back to it sooner.
I spent 1.5/2 years trying rehab (in an admittedly not very dedicated fashion), which produced improvements, but then would go back to almost square 1 when starting running again. Eventually I got an NHS MRI which revealed I had a bony spur poking into my patella tendon. Then had surgery to remove dead bits of tendon and bony spur, and to try and mesh remaining bits of tendon back together again.
The physio advice I got was not very good, was much too intense for the level of injury I had.
If I magically went back in time I would:
1. Recognise that the NHS wasn't the best route to go down. At the time I didn't really realise that the NHS is not very good for non-major sporting injuries. Would pay for a private assessment and MRI.
2. Would use the quad extension machine in a gym. I needed very light loads when I started rehab, which was hard to achieve with bodyweight exercises. Quad extension machine allows very light controlled loading.
3. Medium/Long term would try and follow kneesovertoesguy style training.
I don't know if any of that will apply to you - yours maybe doesn't sound quite as bad? In which case I would recommend looking at kneesovertoesguy style training, particularly the split squat.