Dave,
Where are you staying, have the girls been on thrill rides before?
If you're staying on Disney, then an afternoon break is a workable idea. It depends on how the kids take it. March is not too hot, so they may be ok if you give them a little "down time" just after lunch. A lot of people will take their families out of the parks for a couple of hours. If your journey time to the hotel is too much then this isn't an option. I found that by having a relaxed sit down lunch ( even if it's counter service) and scheduling in a quite 60-90 minutes after lunch it was enough to recharge the kids batteries. There are a number of rides/shows that are indoors and seated. Muppets, Little mermaid, Beauty and beast show all @ MGM. Hall of Presidents, Timekeeper, Carousel of Progress, Tikibirds all @ MK. It's tough to be a bug, Lion King show, Pocohontas show, Tarzan show all @ AK. HISTA, Mexico ride @Epcot are things that immediately spring to mind as attractions that give the oportunity for a little rest while lunch goes down for everyone's energy levels get back to normal. My kids used to work ok as long as we worked in a full restday (i.e. pool, a waterpark or shopping) every 3rd day or so. If your tickets are such that you're only getting for example 4/5 full days in the Disneyparks you're not going to want to waste 4 or 5 hours out of each of those days by driving back to your hotel for an hours nap ( particularly if your kids don't need it). Some kids need a complete break from the parks during the day, some just need a couple of relatively quiet hours in the middle of the day, some are like the ever ready bunny and don't stop AT ALL. The only way you'll find out is by trial and error as to how they work.
There are a lot of rides that different people think are "over rated", however the only way to decide that for your family is to do as many as you can. There are few attractions that won't hold something for someone in your group, so if you personally don't get much out of it, try to watch the reaction of your partner/kids to see what they are getting from it. It can turn a tedious 3 minutes into a magical one if for example, your youngest gets a real buzz from something as simple as the bubbles coming down in the little mermaid show.