There's a theory that damage to tendons causes fibers to stop running parallel and cross-link, causing it to become less liable to further injury, but less elastic (and that leads to a decrease in function). A study found that 90 continuous days of intense self-myofascial release had a 100% rate of disrupting this, allowing improvement. I've been doing it for 90 days.
My achilles tendons will never be right, but stage one of rehab is done. The next step is reorienting those fibers by stretching under eccentric load. Again, there's a study claiming 100% success after 120 days of doing this twice a day (with added side-lying hip adductions, for some reason); I have another 30 days to go on that, but it also seems to be working.
The third stage of rehabbing is strengthening the area through balance, agility and plyometrics. Worried that I'd rip something if I dove straight into that, I've done some basic light jump training. I have a long, long way to go. As a teenager, I had a vertical leap of 21.5 inches and standing broad jump of 7'9". Today I can broad jump 4'7" and the vertical is so embarrassing I won't state it.
I'm back running, though.I can only manage a few hard miles and "hard" is now two minutes per mile slower than it was a decade ago.
It's better than where I was 90 years ago, though. A lot better.