There is that ^^^ . you will not be as fast , just because you are sitting up higher ..
with the summer day being longer, a 60 ish mile day is reasonable, just start earlier ..

I did many on my camping bike tours , and I only average 7~10 Mph.

Maybe a Sram Dual Drive , the triple crank is inside the hub , and the cassette is on the outside.

and the Internal 3 speed downshifts when nearly stopped , which a front derailleur struggles to do.