For weeks I was convinced that I had an Achilles tendonosis-type injury. That changed on Friday when I visited M.C. Dion, a supremely talented physiotherapist located at McGill's sport medicine clinic. Though I suspected there was something different about this injury compared to the time I (actually) had an Achilles problem, her visit put any doubt to rest. After a few minutes of prodding she confirmed it was the muscle/tendon behind the Achilles causing problems: the Tibialis posterior muscle, or tib-post for short. When M.C. added an electrical current to the muscle, the same tingling sensation was present as when it first started healing (slowly) on its own. A day after her visit it already felt better (though it still needs time to heal). Were these the middle ages, I imagine physiotherapists would be suspect of witchcraft sooner than doctors.
Anyhow time to start the real work: doing a lot of heel raises, one-foot balance exercises, and walking drills (i.e. standing on your toes, heels, inside/outside of your foot). There's this counter-intuitive issue of keeping small muscles strong. Because they are small, they must be constantly worked (you might think they will get worked while supporting heavy lifting of the big ones). The catch is, few exercises strengthen these muscles directly. A tiny muscle like the tib-post will never bulge with might like a pair of biceps or quads. It feels like nothing is being accomplishing while doing calf raises compared to a satisfying dead lift. Also something big like quadriceps can be damaged in one part while still function in another, whereas a pencil-sized muscle is a little more binary, which explains why it gets tired so fast when I try running.
I read from Sweat Science today that eccentric calf exercises do in fact make for stronger tendons even in the very long run (i.e. 5 years). Good to know; I already feel the small, supporting muscles around the ankle area being used more than for, say, an easy run. It still isn't clear to me where the dividing line lies between total rest versus when it is time to start applying external loads for a damaged muscle. So much to learn, so little absorbency in my brain.