I'm going to disagree with the Fit. It gets good mileage, but if you're driving that much, I expect you must be on a highway for some of it.
For a lower price range, I'd pick a Toyota Corolla, for 2 reasons. First, with a manual, it gets BETTER highway mileage than the Fit (35 vs 33), and you only lose 1 mpg in the City with the Corolla (26 vs 27). The second reason I'd pick the Corolla is that it is a more substantial car. It will be easier and more comfortable to drive at 65+ miles per hour.
If you are willing to pay a little more, the Prius is the obvious choice -- 45 mpg highway, 48 city. It is spacious, comfortable, has a decent back seat, and there is plenty of room for storage with the hatchback design. It drives great at highway speeds.
Note that dealers have lots of Prius models on their lots right now, but gas is heading back up. There are incentives on them now, but if gas goes up another $0.50 per gallon, watch the incentives and inventory disappear.