Well, it's not necessary to expand the size of the whole world, just put the destination further away on the coast.

Say, moving NY City up to Boston, or even Maine so the trip from DC takes longer.
Unless you are going to include a map, there is no way for the reader to know how far away the destination is.


The tech on your world may just allow for faster airships.

I am confused though,

"The ships as I have written them are too slow,"
contradicts
"slowing down airships (but they still need to have some advantages over sea/rail)"

Ways to discourage rail: Poor terrain, a greedy government that taxes them too much, expensive fuel (poor terrain would probably be better)

Ways to discourage sea travel - expensive fuel, not enough raw materials to build good ships, very rough ocean areas, poor coastlines for ports.