I suffer from the very same problem myself. I thought I'd share my experinece and what has helped me and hopefully it will do the same for you. I began suffering from achilles tendonitis about 1 1/2 yrs. ago. I did go to a sports medicine physician who suggested that I not do the very activity that aggravates it (which, of course was step-of which I refused to give up-I LOVE Cathe's workouts too much!). He also recommended I go to physical therapy, of which I did but quit after the 1st session. I then began doing some research of my own and I've come to learn that icing my ankles both after working out in the morning for 20 minutes, and again at night for 20 minutes has made a HUGE difference. I also vary my cardio workouts. I do not do step every day like I used to. I now alternate doing step with the stairmaster and bike one day, kickboxing another day, etc. I no longer have pain like I used to and am back to enjoying my workouts again. However, I am afraid to ever stop my icing ritual, for fear of the pain returning. In addition, I, too was working out for a while on a concrete floor in my basement that I layered with padding and carpet, which I believe was a huge contributor to the problem in the first place. I then began working out in the family room which has wood flooring with carpet on top. I believe the wood flooring 'gives' when jumping around, whereas the concrete floor does not. I also bought sport heel inserts for my sneakers, (which can be found in any drugstore where they sell the orthotic products-just ask), which I believe has been a tremendous help in alleviating this problem. I don't know if any one particular change I've made that did the trick-I think it's the combination of everything.
I hope this helps b/c believe me, I know how frustrating it can be to have the passion and the drive to continue working out, but be in agony and not enjoy your workouts b/c of the pain. Just hang in there, try to find what works for you and stick with it! You'll be so glad you did!! ;-)
...Carole