When I began several years ago I started with an old beach cruiser I bought 20 years before sitting in the garage for the past 16. I rode that for 9 months before I bought another bike. Just wanted to make sure I didn't spend a lot and then quit. My next bike was a Raleigh c500 ($550) that I rode the he** out of. Since Stillwater OK doesn't sound like it has huge hills, I'd suggest just getting a bike that looks like it has some big honkin wheels on it and ride it till they fall off. Once you start getting into it, you will have a better idea of what you want and time to save up. If you can find an old industrial bike like a Worksman it would be awesome. There is a guy around here Stephen H who had one and rode it for more than one would expect. You might PM him.
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Be watchful, stand firm in the faith, act like men, be strong. Let all that you do be done in love.