There is only one way to fly trips longer than one could drive in the same length of time: carry something you can wee into.
The red plastic bottle carried by the usual pilot shops is good, and it also has a "gurl adaptor" available which also works although the lady might wish to climb into the back seat (not for privacy as much as that she needs to be more vertical when using it).
Personally I carry a few empty Tropicana plastic juice bottles. They have very good screw tops and never leak, even when you part-fill them at FL160 and then descend, with the pressure change crunching the bottle. One just chucks them in the nearest skip. They can be used while sitting down; it's even just possible to do the radio.