There's little to add to the excellent responses already given, but I will say that the OP reminds me of me less than a year ago.
I took the plunge with an inexpensive mountain bike (thinking I could/would do both trails and roads), and my first ride of 5 miles was TOUGH. But, as several people pointed out, what absolutely floored me was the FUN. I'd completely forgotten how much fun riding a bike can be.
After that, it was ride for the fun, and set some goals, and see what could happen. I'm no athlete and never was. I've quit more exercise programs than Carter has liver pills, after the initial enthusiasm wore off. I maintain a $99 a year gym membership simply because it's cheaper than starting all over again somewhere, but i hardly ever go to the gym.
But that inexpensive bike (I didn't want to spend too much in case I quit this too) has been a blast. In the beginning, I'll admit there were days when I had to talk myself into getting out there. But they are few and far between now -- in fact, it's the opposite. I'm cranky when I don't get to ride.
I've pleasantly surprised myself at what I've been able to do, like "ride my age in miles for my birthday," and I'm far healthier now than I've been in a long time -- BUT -- for me, it's still all about the FUN factor, for the most part. Swooping, gliding, riding no hands, exploring new neighborhoods, wearing myself out on a long weekend ride, all of it is fun. I'm a kid again, on my faithful steed.
So...I guess I did have something to add afterall!
Anyway -- just get started, hang out in this forum, and have FUN!