I had pretty bad achilles tendinitis a couple times–my symptoms included great pain in the area (lower calf) while running, and an odd pulsy-creakiness to the area when I was not. It would throb and audibly creak, which was disturbing. Overall I attributed the injury to both increasing my mileage too fast and from running too long on old (too many miles) shoes.
What helped most was taking time off, icing, and stretching it.
At night I would wear The Strassburg Sock as long as I could stand it (it’s slightly uncomfortable on your toes after awhile), and during the day I could try and stick my foot in positions that keep the achilles/calf stretched (similar to how it looks on the Strassburg Sock pics)–having it stretched increases blood flow and promotes faster healing. It would always feel a lot better after being in that position for awhile.
Since then, I think transitioning from a heel strike to a forefoot strike has helped me from getting injured too (less overall impact), but increasing mileage slowly, not letting my shoe mileage get out of hand, lots of cross training even while running lots, and really make sure you’re recovering right (diet, icing.. I ice 2-3+ times a week even when i’m not sore at all, just for precaution).
This is not an injury I would suggest trying to run through–it’ll heal much quicker if you just lay off and treat it right. It’s an injury I’ve heard can come back again and again, but I haven’t had any issues for several years after now, which I attribute to taking better precautionary care.