So, Mr. Fenn also published another book TFTW wherein a lot has been made of this clue. People have discussed in various forums the mathematical solutions and other metaphysical outcomes for this phrase.

In perfect Fennian fashion, I believe this clue to be nothing more than a directive to go for a distance to the next clue. In my solution this says go a short distance on the map to the next clue. People have made this more complicated than it actually is. Mr. Fenn has created an entire mythology around this phrase, which has the simple meaning:

Continue on the Green River, not very far on a map, until the next clue

Through Utah, Enter Colorado...

See, it isn't really far on the map...