Hi Jennyl,
I can understand your concerns and I know it's easy to just say 'you'll be fine' etc when it comes to actually physically driving it can be nerve wracking! Some drivers are more adept to switching and other's maybe less so, I'll assume for the sakes of the post, that you are less so and are concerned becuase of it.
First thing is - try not to worry too much becuase from what I remember the roads are quite open and large - at least around the Disney/orlando kinda area. We all went as a family and my sister was in a similar situation to you - and she had to drive us all about in one of those 12 seater massive Dodge Ram thingies!
I remember thinking wow, the roads are so open and large that it's quite easy to 'build up' confidence - which is exactly how my sis coped with the change. So, I suggest when you pick up the car, maybe if you stick some of that cheap petrol in and maybe pre-plan a route, you can stick to the larger roads to help build up your confidence and familiarity. Then, if you do come across smaller roads you'll feel much more confident in using them.
You can put yer other half to better use by makin him be co-pilot whilst you do the initial confidence build up thing!
Hopefully this little plan will go someway in helping you adjust
Cheers! DtM! West London & Slough UK!