There's lots of things to work on when you're just starting. The "peddling like hell" might be a good thing if it means that the rpm of the crank (cadence) is high. Like most beginners I started with too low a cadence (65 rpm) and thought I was a natural "masher". I read where 80 was a more efficent cadence and worked on "spinning". I now spin in the 90's without thinking about it and exceed 140 in my spin class at the Y. I've only been riding since Oct 2010.
If you had multiple gears where you could choose your cadence for a given speed. One could gear up and lower cadence which would work the leg muscles harder. Or you could gear down and increase cadence that works the cardio system harder. So "peddling like hell" if I understand your post means that your leggs/knees are getting a break while your cardio system is working harder. I feel the cardio system is a better system to work on first. Without going into great detail I also understand that for endurance that a higher cadence (to a point) allows for longer rides and better recovery between rides.
My point to all this is don't be in a big hurry to move on to 2 wheels. Lots of good miles in your trike.