Eccentric heel drops. 3 sets of 15 reps. Ideally twice a day. There are tons of youtube videos that will walk you through them.
What happens with tendon injuries is they take a long time to heal, and they build up scar tissue. Lack of blood flow is the biggest issue with the slow healing and dealing with that scar tissue. So what most likely happened is you healed your achilles injury. In the time off you probably healed it even more and built up more scar tissue. You'll need to get used to using the achilles again in the proper way. The heel drops will help with that. So will additional running. Keep the paces really easy. But for me, I found that running daily was easier to adapt to than taking days off.
The other thing that I've found helpful is massaging the tendon. Get some lotion or oil and in the evenings rub along the tendons. Work on any rough spots you feel. Not hard. Just work lengthwise along the tendon. Think about smoothing it out. During one of my longer achilles rehab sessions my PT said basically what happens is the body knows it needs to heal a certain part, it sends all the new cells in a dump truck and they get dumped on the injury site. This patches everything up, but you need to smooth them out. Gently. Smooth, even, gentle rubs.
So this is the part where I also say consulting an actual health practitioner could be a good thing. Everything above is my personal experience. I'm over twice as old as you, and I've had a lot of achilles issues. But getting them in your late 30s might not be the same as getting them in high school.