You'll probably get a better answer if you give everyone a little more information.

I'll share some of my own experience to give you an idea as to why. I started riding again about 3 years ago. I rode as a teenager on a road bike, but that was many years ago. When I started again, just by happenstance I got a 1980's vintage mountain bike (no suspension, pictured below) at a garage sale. It was a great bike for me at that stage. I was a few pounds heavier, and this bike was a tank, with a triple up front and very low gears in the rear. Just what the doctor ordered for someone as out-of-shape as I was. I rode that bike for 1400 miles, all on the road, and dropped a few pounds. Then I got a used road bike, and rode that for a while.

My point is, had I started on a road bike, I wonder if I would have persisted. The gearing on my used road bike was not user-friendly for someone who was in as poor shape as myself when I started to ride again.

So that's why it would be helpful to know a little more. In my own case, I started with riding around my local roads, gradually lengthening the distance until I could ride 20-30 miles at a reasonable speed. But there is no way I could have ridden the Loop when I first started. Everyone's different, though, and probably you're not as out-of-shape as I was.

In a nutshell, if you're out-of-shape, you may want to get started with a hybrid bike of some kind. The upright riding position is not as demanding as the riding position of a road bike. If you're not terribly out-of-shape, consider a conventional road bike. Get a mountain bike if and only if you plan to do >95% of your riding off-road, say, on the Loop.