The Log is in Current.gpx and NVRAM
EveryTime the Nuvi Boots Current.gpx is created anew from NVRAM.
I think what you are trying to do is not possible in the Nuvi range.
What you can do is open the current.gpx in basecamp and convert track to route with many points and then change the points to off road in trip planner.
In this way you can sort of create an off road route that is sort of suitable for mountainbiking etc.
Otherwise just buy an outdoor device. (LOL)