Originally Posted by RT Firefly
This appears to be the closest answer to my dilemma. After digging around a bit, I discovered that my new fish finder has an odometer and trip log. That's about the last place (actually, exactly the last place) I thought to look, but there it is. There doesn't appear to be any way to give it an offset to account for previous miles, so I'll have to tally them up manually and mentally add them.
Thanks everyone for inspiring me to look a little harder in front of my nose....