The course was certainly created manually. The red line follows the roads perfectly (you can see this if you zoom a bit in), a GPS watch is never fully accurate.
Good observation. My question remains if he drew it before or after he ran.
If before, did he cut his own course that he drew? And how would he remember which turns to make in Hyde Park? Even if you've been there a lot, there are quite a few different options to take across wide open grass.
If after, did he add some extra loops or distance to get it over 26.2? Also if after, how did he know when he had run at least 26.2 miles?
I still haven't figured out a plausible scenario to account for this run.
Initially, I thought maybe he drew it this way, but then cut the loops in Hyde and Regents Park to the barebone while running. I don't know.