From the Hertz lot (they are not on the airport) you need to go back south towards the airport, and then take 528 East (also called the Beeline Expressway) to where it joins with I-95 South in Cocoa, and then take 95 South. The Beeline is a toll road, and will cost a buck or so to get to I-95.

Some people might try to steer you over to take the Beeline West then the Turnpike South. This has numerous diadvantages. First the cost is higher, second the intersection from the beeline to the 'pike isn't good, and third, you would have to get off at Yeehaw Junction (a.k.a. Florida's Redneck Capital) and take 60 to Vero as the next 'pike exit is all the way down in Ft.Pierce