DBSA, if the cusshioning has has gone in your old trainers and the heal will be lower than ideal thus putting strain on your achilles. You could try putting a heal wedge under the insole, this might aleviate the pulling on the tendon.
I've just had my 1st physio session for the same thing, he seems to think 2-3 weeks for recovery with just cycling for excercise with some light stetching and the use of a heal wedge in my work shoes during the recovery period.
Good luck
Jenks