I just do "eccentric stretching" you rise up on your toes and slowly let yourself down. It's it's own exercise, too, so you could use it as your exercise routine until it becomes second nature! But don't hurt yourself, definitely work with your doc to ser what you should be doing. If you push too hard you could injure the tendon and that needs surgery sometimes… occupational therapy is amazing if you find the right one. And, of course, "rice" Rest, Ice, Compress (ie compression socks) and elevate!
I think initially best to rest, can even use ice pack if pain came on suddenly, then in the next day can switch to warm compresses… stretching helps and then gradually return to previous exercise activity but go slow. Important to make sure it is tendinitis and not some more serious or painful injury such as a tendon rupture or tear in muscle.
Try to stay off of and complete exercises that will not affect the lower foot area but like swimming or low impact indoor cycling, You can do it.