It seams that one of my questions has yet to really be answered: Renting a tank for a day or so and just breathing through that to get used to breathing through a regulator, is it a viable way to help improve my SAC? I have seen that I breath harder when on a regulator than not.
The problem isn't breathing through a reg, but instead breathing underwater. You have received a lot of valuable insight here. Let me try to put it all together in one post for you.
TS&M gave you the answer when she told you the major issue is being comfortable underwater. For new divers, a huge degree of discomfort is simply being submerged. We are land mammals, and it takes quite a lot to get over our instinctive fear of drowning. Subconsciously you know you are in an alien environment far from the surface (different psychologically from swimming on the surface or slightly below it), and your body goes into stress mode. When this happens, your heart rate increases, your body tenses, and you breathe quicker. The result is a rapid depletion of air supplies.
To reiterate what TS&M has told you in the bit she quoted in her post, breath control is not about breathing through a reg, but more about relaxation. Breathing "deeply" means using the diaphragm to draw air in and push it out fully, not just pumping up the top half of the chest. The breathing impulse is a function of the amount of CO2 in the body, and by exhaling completely after each inhale you help to avoid creating dead spaces in your airways where CO2 build up is possible. Any part of your airway that isn't "flushed" contributes to CO2 build up, so do breathe "slowly and fully" as recommended by Scubagolf.
Other biggies in terms of "air-hoggism" are not swimming in trim and proper weighting. Think of the angle you hold your hand on the surface of the water when you want to splash somebody--about a 45° angle, right? The energy you put into the movement is transferred to the water itself so that it splashes up--and your hand slows down. If you use a flat hand, parallel to the surface of the water, you don't get the splashing effect because the energy you use doesn't transfer to the water itself and your hand just slides through. Now imagine that same positioning underwater. If you swim out of trim (usually with legs dangling down), in order for you to move forward you need to push a lot of water out of the way, and of course that takes a lot of energy. If you are horizontal in the water, you push a lot less water out of the way, meaning your energy is used mainly for your own propulsion.
Related to this is weighting, as Seaducer points out. If your legs dangle down, you will be finning yourself upwards at an angle rather than simply forwards. Divers who dive in this position usually think they need more weight because they are constantly rising in the water column, and then when they put more weight on the weight belt, their legs drop even further, meaning they become quite over-weighted. When this happens, these divers need to pump a bunch of air into the BC to get neutral, even at shallow depths, and they then end up with buoyancy issues. If you have the right amount of weight distributed so that it allows you to swim horizontally in the water, you work less to move yourself through the water more efficiently, and this results in improved air consumption.
Also as Bryan mentioned above with his barbie illustration, another important factor is fining technique. Bicycle kicks get you nowhere with a great deal of effort. The "barbie" image above with extended legs and pointed feet will produce an effective flutter kick (as long as you are horizontal). There are other kick techniques as well. I find that frog kicks are the most efficient for me. When I count kick cycles for 100 yard distances using both frogs and flutters, I kick about 1/3 fewer cycles with frogs. Imagine the savings on air consumption by not needing to use those long muscles of the thighs so much, as Lee noted!
The whole thing is like a tapestry with different aspects woven together: relaxation, breath control, weighting, trim, fining. This is why air consumption is one of the big frustrations of new divers--there are so many different facets of it to work on that it takes a while to get it all coordinated.
You CAN get there, though! Most of us have been in the same situation you are now in, and we have done it.
