Assuming you are not getting formally removed from flight status, you'll be ATP eligible at 750 hours... any hours, regardless of where you get them. So you could do any GA job to get there, or get a CFI and do that for a while, and the off to the regionals.
You'll *probably* be in better shape for getting to the majors than the average civilian/regional pilot, but that does depend on the circumstances of your leaving the mil early.