I considered the PTA route at one point and I have a bachelor's degree completed-- along with all of the PTA pre-reqs for the program closest to my home.
The program near me is a 2-year program regardless of whether or not you have a 4-year degree. *NOTE* If you do not have classes like Anatomy & Physiology, Medical Terminology, English or Math done, you are technically looking at 3-years for the PTA program... as most applicants have these type of classes done before applying to the actual program. It's exactly the same scenario that an English major would encounter when applying for a PT program. They have a B.A. but they do not have the pre-reqs, so they need to take an extra year and do pre-reqs before applying to a PT program; hence, the 3-year program "technically" becomes a 4-year PT program.
When you are accepted into the program, you move through it with a cohort. I think you will find that most two-year applied science programs are like this (PTA, RN, Sonography, etc...). You can't skip semesters or move through it any faster than the person sitting next to you in class who has no degree. For the sake of clarity, you might have something like PTA Practicum I, PTA Practicum II, PTA Practicum III and PTA Practicum IV-- all of which you do in order, one per semester, with your cohort.
Some of your classmates may be completing "pre-req" type courses during the first semester (the program near me allows for the completion of humanities and social sciences courses after being admitted), while you will already have those done. So, perhaps they might be taking 16 credits and you might be taking 13.
All of this being said, if you already have a degree in exercise science (and probably have all of the pre-reqs for PT completed), you might want to strongly think about going PT and not PTA... it's just one more year (but unfortunately, a lot more money).
HTH