I've had both "regular" and insertional Achilles tendonitis (probably tendonosis at one point). Here's what worked, in rough order of effectiveness:
- put heel lifts in your shoes. The thicker, the better.
- do all of your runs on flat surfaces. No hills, no uneven terrain (grass), no banked surfaces (roads, some indoor tracks). Do all of your runs on a treadmill with 0% incline if you have to.
- eccentric heel drops with a weighted backpack. Drops with a bent knee are good for "regular" achilles problems, but for the insertional type, stick to straight-leg drops and don't go below parallel.
- heat your Achilles before runs, and ice them afterwards.
- cut your mileage in half. Intense workouts may be fine as long as you don't do any all out sprints.
I fully recovered and set some PRs but it took about 3-4 months to fully get over it.