Buying used can be really good, but if you're not comfortable with that, don't be afraid of entry-level bikes in your price range. As the previous post said, often the components change but the frame stays the same as you move up. And even low-end components work pretty well these days.
I think (others disagree) that we think too much about weight. Ten pounds matter, but 10 ounces don't. Be sure the bike fits, and learn a little about gearing. Many entry- level bikes are geared almost like racers, too high for casual riders.
The truth is that you can do 20 miles on any bike, and if you're riding solely for fitness, it doesn't matter if you have to work a little harder. OTOH, riding a nice bike is more pleasant than riding a sled, and someday you might want to go 50 miles. I've never been sorry when I went one step up from what I really needed.