Well, that's great for the Clearwater National Forest! Seems like that data only covers that forest, not the whole country.
I couldn't see exactly what we're looking for on the NPS site, either, but keep looking.
If the data on Clearwater is in shapefile format (.shp + some other files) then all you'd need to do is convert to GPX. There are a few programs out there ($$$) to do it, or you can wait until we add shapefiles to TF. I've got export to shapfile (tracks) working so far...