Hmm, the maths I'm trying to work out is the ratio between sealed road, dirt road, and dirt single track. I'm thinking something like 40% more effort for dirt road, and maybe 100% for single track. So 25km single track, 36km dirt road, or 50km sealed road are all the same effort. Just wild guesses at this stage.

Someday I plan to seriously analyse some GPS traces to get the answer to the OP's question though.