10MPH is a bit high, regardless you spent 2.2 hours to figure this out :P good job.
What could you do to find the actual sprint speed? Find a server that shows the range at which you kill someone and time the run from start point to death point?
I guessed. Average footspeed over distance for humans is probably closer to 8, but since running in Rust is constant and you don't have to stop - at all - if you don't want to do so, I bumped the average up a bit. That was my quasi-scientific analysis.
Edited:
On the north end of the island is a beach that stretches for at least 5-6 miles ... and is probably half a mile wide. There was a channel cut that looked to me like where there might be a plan to have a flowing body of water through the heart of the island. Other red herrings like a sinkhole, a lone bear all by itself in a tree, 10 mi outside of the developed area, other cool stuff.