55mph rural roads are hard. I seriously burnt myself out one summer commuting on these roads for about half of a 20 mile commute, 2 to 4 times a week.

I don't typically pull of the road for anything (though I am on two wheels, not three). What I do is make it painfully obvious to the overtaking vehicle that they cannot pass using the same lane. I will "take the lane", meaning I'll get out toward the middle and many times on these types of roads I'll give a signal with my hand to "stay back" (put my hand out, palm toward the driver, and wave them away).

This works most of the time. The watch word is to be assertive. Act like you own your own space.