If you can afford to spend about $600 then it would be worth it to visit a local bike shop or two or three. I like Trek and Cannondale but there are others that are good, Giant is another one. Talk to them and let them inform you. You might think well this is a lot of money to spend and it is. We are all frustrated with the cost of this hobby. I mean buying the bike is just the beginning of the expense. I mean I have about $400+ in electronics on two of my 5 bikes, a $200 repair stand and quite a few dedicated bike tools and spare parts. Then there is clothing and helmets. And you thought it was as simple as just going to the department store and telling the clerk "I want that one".
Obviously those of us that post to a specialty forum are going to be more than a little more than just casual. So true, we tend to think big on this topic whereas you might have a much more modest plan. What will happen is if you obtain a good solid bike that fits you properly, you will find this to be a most enjoyable past time. If your bike is ill fitting or prone to breakdowns or is just plain old heavy, you will ride it once or twice then that will be it.
We don't generally give our kids expensive bikes for numerous reasons. When I was a kid I had a department store bike and loved it. But then i got a paper route and thus earned some money and one of the first things I did with that money was buy a "real" bike. Even at the tender age of 13, i quickly found out that money spent = performance realized. We as adults especally as we begin to get into shape are not going to keep at it with equipment that is, to be frank, a pain in the tush.