I started with a 5.5 mile commute. Honestly, I didn't even try for a few years, although I was in my 40s when I re-started cycling. But 5 miles was a good trip to take my girls down to a bookstore (with coffee shop) or ice cream store. After a summer or two of that, I started branching out.

I'd suggest you start slow. Aim for 2-3 times a week (TT or MWF, maybe?). Give yourself permission ahead of time to drive in or take transit if you don't feel like it. I'd guess you'll start feeling like it a lot more after a couple of months.

Make sure you get slicks (tires without tread) if you start on a mountain bike. You should be able to get a decent MTB for $300 or so at your Local Bike Shop (LBS). Slicks cut the rolling friction a lot, which means you can ride faster with less effort. It's why many of us end up on road bikes of some description (touring, 'cross, racing, etc.), because knobbies slow you down.