For public transportation, you described it perfectly: Cheap or fast, but you usually can t have it both ways. The exception is between large cities. For example, the bus between New York and Washington DC is usually very fast and goes direct. For travel in smaller towns, renting a car is usually the best way to go. Car rental is usually much cheaper than in Europe, and gas (petrol) is cheaper too. Often you can rent a car for around $30 a day.
Americans pay a much lower tax rate than people in Europe. As a result, public, subsidized services such as trains are not as efficient as the ones you will find in Europe. As one of the other people said, a rental car is probably your fastest and most affordable option.
Buy a cheap car and when you are done traveling sell it off. That is what the guys of Top Gear did.
Rental car.