I think you might be misunderstanding the way I would use such a feature were it to be added to TM.
Before hitting the road, I often grab a screenshot of the area I plan to visit. I then print it out and jot down the mileage along all the routes visible. I would omit the routes I've already driven unless I think I would need to traverse them in the process of covering previously undriven routes. On a recent trip to Jacksonville, I took over a dozen such printouts with me, all calculated and labeled by hand with routes and mileage I was thinking of driving.
The attachment is an example of a screenshot from TM near Ashland, OH when I was heading up there last fall. After grabbing the image, I would have written in the route numbers and mileage between junctions with particular note of the untraveled portions so I might be able to figure out in advance what I wanted to cover.
As noted originally, I currently do this by adding up the mileage of each segment. All I'm wondering is if there is a faster way or if one could be added. If not, I'll keep doing it by hand. No problem.
On a side note, I use Google Maps frequently when on the road and review/update the "Your Timeline" feature.
As for the accuracy of the mileage, for my purposes, the TM approximation is sufficient. It's more of a guide as to whether I want to drive a couple miles out and back or dozens of miles. Sometimes it's not worth the time commitment just to clinch a road. Other times, getting that clinch might justify the drive.
