New bike advice...

Check out the search function, this has been discussed many times with many different brands and types discussed that may help you out.

I just picked this up yesterdy. It's a "cyclocross" bike. Basically a road bike with slightly bigger grippier tires so you can ride on trails, gravel roads, etc.. This one's going to be out of your price point, but they may have some cheaper alternatives. I got this as my first real bike to excercise with. Took it out on the road, through grass, and through some muddy areas with no problems (other than me being out of shape). It really has a lot of upside since it has so many uses. Only issue is that cyclocross is apparently a very specific sport and there are some serious competitors so the price of the bikes starts a little higher than road bikes on average. In almost every bike shop I went into, the sales person said that they used that type of bike as their commuter.

It depends on exactly what you want when you say "exercise/longer ride purposes."

A road bike with the drop down handles is going to put you into a certain type of "form," if you will, and is light for speed, aerodynamics, etc.

A mountain bike will obviously have thicker tires to be able to handle off-road conditions. A traditional road bike tire doesn't handle off-road situations well at all.

There are hybrid bikes which are pretty good. Fairly lightweight, regularly bars so you're not in drop down form (more comfortable), etc. I'd say that's the most common "commuter bike," other than maybe the cruiser. And well, assuming you're a man, I don't recommend that.