i'm hoping to be a CSCS by early next year so i'll throw in my two cents as to what my plan is gonna be.
after getting certified, the first thing i'm gonna do: nothing.
now i don't mean nothing nothing, but nothing really with clients on a paying circuit. do research for a year, go to conferences, network. get some guinea pigs to try out building programs for based on individualized goals. maybe even go up to the local high school in my city and ask if they want some volunteer services in working with the athletes for various sports. also spend time building a business plan (i've got one started now, but sorry, i ain't giving away my ideas

).
then, after the first year, year and a half, maybe start doing some work where i actually charge people. but, do it on a very part-time basis. then see if i'm marketable enough to where over the course of 3-5 years, can dump my day job and go into it full time. other goal before dumping the day job: an MS in exercise science.
bottom line, if you've got a full time job right now, stick with that, don't make any harsh moves. take a change very slowly and spend some time coming up with a busniess plan and building yourself up in the part-time market first to see how it goes.
good luck.