Another issue you might want to at least consider before putting in this
kind of "unpainted, untreated...rough sawn lumber" is possible pest
infestation issues.  We recently had a big problem with carpenter bees
inhabiting an untreated wood deck at our museum.  Though this type of
bee is not especially dangerous to humans they do look very scary, we
were also concerned with possible movement of the bees from the decking
to some of our wood objects.

The info. I was given, at the time, about carpenter bees states teh
following: "...will nest in a wide range of woods, but prefer weathered
and upainted wood....Carpenter bees rarely attack painted wood.  They
can be discouraged from using wood by applying an appropriately labeled
repellent material such as one of the pyrethroids adn microencapsulated
or wettable powder formulations are best"

These bees may not be a concern in your part of the country or for the
kins of wood you are using but it might be worth checking with a pest
control consultant prior to your investment in the new walk ways.

Sylvia S. Duggan
Assistant Registrar, Loans
The University of Pennsylvania Museum of Archaeology and Anthropoloty
33rd and Spruce Streets
Philadelphia, PA  19104