The easiest way to get to Key West is by plane. Many airlines have affordable flights to Key West and the airport is modern and clean. You will find plenty of taxis and ubers outside the terminal so getting to your hotel is no problem.
If you must travel by car, it can take between 3 and 7 hours from Miami depending on traffic. When you see the 7 mile bridge, you know your getting close.
You will find that walking or renting bikes for a few days gets you anywhere you want to go without having to deal with the frustration and expense of parking your car. In Old Town there's limited on-street parking.