I rode MTBs on the road, commuting (pre- and post-car at age 16), and fire roads. I literally had no idea there were any singletrack trails nearby. Sad, huh?
Then for a lot of years I didn't ride at all. Got fat, lazy and very out of shape. In 2010 I started bicycling again and I did a ton of research. But there's only so much you can know on paper versus things you learn getting out there and doing in.
Anyway, my biggest mistake was buying a Cannondale Quick CX Ultra. It's a relatively expensive hybrid, and money that would've been much better spent on a real CX bike or a decent hardtail MTB. I did some longer rides on it on the road and tried various tires on singletrack trails. The Cannondale Super Fatty Ultra headshok fork might have been state of the art in the late 90s, but it sucks today. Don't even think about trying to climb or sprint without it locked out! And I'm sure some of you are skilled enough to ride my local singletrack trails on cross tires, but I wasn't. (still am not.
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Since making that purchase the first thing I did was get a real road bike, which I log the most hours and miles on by far. And now this summer I finally got a real MTB, building up a used SC Blur frame with new parts. The Cannondale isn't totally wasted by any means because now it's a rain and winter road bike, and I might even do some CX races with it.