There is almost nothing about trip meter in the ops manual, built-in e-guide and just a tad in the installation manual. While a simple function, I am really surprised it isn't documented better. I will let them know about the lack of information. It was likely something that was added later via software update and never got documented.
I would recommend setting a "Trip Meter" box into your slide out data box screen from the left side of the screen. When you go to leave for the fishing trip, press and hold the trip meter box and choose the reset option to zero the value out. Take the trip and the value should grow until you are finished. At the end of the trip you can note the value and do it all over again the next time. If you wish to set limits to the max trip value and be notified you would need to turn on the trip meter and set a value under the notifications area of the menu.
If setting up an instrument page a trip meter can also be setup there and be as big as full screen if you wish. The operations and how to reset it are the same as when doing the NavData box.
Chapter four of the ops manual covering marks and points explains that part better than I can.