This advice is only good for me...with that said...
I started off with LBS purchased bike that did not fit. During the first summer I put 1,200 miles on the bike. I thought that I was doing well. I lost 15 pounds and felt terrific. The next summer I found a used, consignment Cannondale that was the correct size for me. That summer I suprised myself with 3,300 miles and lost another 10 pounds. I rode the correctly fitting bike for 5 more years before I got a very nice Cinelli that was professionally fitted to me. Over the next few years the bug bit me and I averaged between 6,500and 7,700 miles.
The point is not my addiction but rather correct bike fit allowed my addiction to surface. The first bike was way too large and was holding me back and interfering with my enjoyment. Enjoyment equals more time on the bike and better use of the device.
Get a used bike that fits and allow yourself the pleasure. While you are riding think in terms of what features and components would make your next bike better for you. Then buy that ideal bike and ride the wheels off.
Okay, the down side...every time you ride "the next bike" you will be thinking of the "next bike". But you will feel good and live longer. Good riding and keep the rubber side down.