When I ran across it, I compared it to the routing function on Paddler Planner.

I did the exact same route and they matched almost perfectly. Paddle Planner has more features but this is free so there is that.

I checked a portage length on PP and this one.
Used a on-line Miles to Meters conversion and they were within a few meters of each other.

Since most of us do more than one portage trip you can do the math for distance traveled yourself.