A marine will probably have an easier time dealing with the doctor egos of the vets than most folks do, so there's one thing in your favor!!
It depends very much on the clinic and the types of caseloads they handle as to what a vet tech's experience is. I worked for VHUP (Veterinary Hospital of the University of Pennsylvania) which is one of the busiest vet hospitals in the world. Even the vet techs were specialists there. I worked as a surgical tech for the reptile and avian surgery department, and further specialized in working at the various Humane Society surgery clinics that Penn used to staff.
So all I did was surgical assist, and it takes a strong stomach for that. It's many hours on your feet and physically challenging to keep your hands steady after many hours of adrenaline-filled work. Again your training may be a plus there. I could perform a number of semi-invasive procedures under the direction of my vet bosses, like placing stomach tubes, starting IVs, draining abscesses, debriding wounds, and even stitching surgical wounds.
In smaller clinics I've heard of vet techs who basically weighed the animals, showed patients to and from exam rooms, fed, watered and exercised the patients who had to be boarded there and not much else.
It's best to try and talk to techs working at each facility to get a better idea of what duties techs are allowed to do.
The Zombie Mama is here!