To get to the rental cars, you'll need to go out of the main terminal on the baggage carousels level and wait for the appropriate shuttle for the rental company you reserved with. All of the car rentals are located in a row along Aviation Drive, about a 5-minute shuttle ride from the main terminal. However, it can be a short hike from one to another if you plan to browse for prices/availability. Especially if you're hauling luggage, you don't want to be hiking in and out of their lots from one to another, so reserve a car with one company and take the shuttle directly there. There is a lot of construction for the Silver line going on along the street in front of all the car rental lots, too, that you won't want to be walking through.

I agree with the others about flying into IAD and driving to Philly. Can you add a connecting flight IAD - PHL instead? If that's not possible, then I recommend you get a hotel near IAD. There are a LOT of nice, inexpensive hotels around here.