I had a similar situation with a different tendon. Mine was anterior tibialis tendon that was injured. For me it came on suddenly about 4 days after my last half marathon a couple years ago. Anything that even touched the inside top of my foot - basically most shoes that weren't ballet slipper style or flipflops (or VFF Jaya/Classic), above my arch would aggravate it, and even the smallest arch support would also make it hurt like crazy - it was the reason I ditched the shoes and went barefoot/minimal in the first place.
Like another poster mentioned - it took about 4 months to fully recover - after 2 months of avoiding the doctor and doing nothing but rest and ice and avoiding shoes as much as possible. Once diagnosed I was referred to a PT - it was the PT that made all the difference for me. I did strengthening exercises daily for 8 weeks, and the PT did some kind of massage that hurt like hell but was intended to break up the scar tissue on the tendon 3 times a week. I started running again at that point, barefoot, 5 minutes at a time and worked my way from there.
Here's some of the PT exercises that I felt helped a lot:
Squats while on a balance board - the board tipped forward and backward, the idea was to keep it level while doing squats.
Standing on one foot on a bosu ball. 1-2 minutes per foot per rep to start, then added time per rep as I got stronger. 3-4 reps per foot. I still do this one at home on a cheap balance disc I got from Amazon, not as big or nice as a bosu ball but more affordable than one too.
Foot lift thingies on stairs. Stand on the bottom step, facing downstairs with heel on the step and toes pointing down. Lift foot and toes as far as you can go (is that called dorsiflexing?), repeat 15 - 20 reps per set. I started with one set once per day and increased as I got stronger to 2-3 sets. This exercise really aggravated the tendon and hurt a lot at first.