My wife just pointed out to me when we drove to the Marriott we encountered much road construction. Hence my estimate of driving time may not be accurate as to driving time.
I would not do the trip without a car. You do not need four wheel drive to go to Marriott but will need one if you are going inland to Rain forest region. Try to plan vacation in dry season. Wet season our summer is very wet.
Costa Rica has a very unusual requirement for rental cars. You must take extra liability insurance and it includes a CDW with rather high deductable. Could use credit card insurance only to cover deductable. Avis actually called my credit card company when we were there to see if I was covered before they allowed me to sign waiver of additional insurance. (Only place in the world that has happened.) Still charged $12 per day extra for liability insurance. I