I have several. I keep thinking I should hit that link for SNM's success stories over in the stories forum.
Older ones....
Completing KM the 1st time I did it. (it was my 1st Cathe and I was about 225lbs at the time) the warmup nearly killed me, the blasts about had me at a 911 (it wasn't pretty) and the legs drills were pure evil. I THOUGHT I was fit/fat at the time, bc I was able to do intermediate tapes by other instructors pretty well. But I was determined to finish it, and I DID!! The next couple of days were quite painful, but I came back for more and more. Now KM is an easier day workout, but I still don't do the leg drills
Lifting weights I never dreamed of! I remember when the 3s and 5s were killer! Now I stand on them for raised toe deadlifts! And I can squat/lunge more than I ever would have figured. One day I realized that I used to carry that much weight around all day, every day and squatted down to get things off the floor, so no reason I couldn't do it in a workout. Weight is weight. So I stopped with the "I can't" and "its too heavy" and worked my way up. It wasn't fast, it was sometimes painful (those special Cathe DOMS), it wasn't easy, but I did it!!
More recently...
Being able to run, I started last summer, but have recently really gotten into it, and now actually consider myself a runner
I'm not fast, but I'm running and having fun.
The biggest things (not sure they're really milestones) is the realization that fitness is fun. It makes the rest of life so much easier. And getting over the mental blocks. I CAN do it. Sometimes it takes some cursing and swearing and stopping and starting, but once I decide I can, I can.
Nan
(do I win cheesy post of the day?)