I suspect this guy was actually running significantly more every day but also took some significant time off.
Russ Cook, who also ran the length of Africa, ran a route that was 2000 miles longer, in about 5 less months. He covered on average about 28 miles per day.
They're both very impressive accomplishments, but not as physically impressive as mentally, at least in my opinion.
Funny, you bring us Russ Cook, his body was literally breaking down. Again, 15 miles on average is tame for a long distance runner. It's starts to become exponentially harder when there are no rest days involved. Both of the achievements are nothing to scoff at.
His Strava log clocks him at double that (34+ miles per day) over the last couple of days. So I'd say the average is not representative of his normal, on-the-road pace - which makes sense because he was detained for 3 weeks, visited extended family for part of the 517 days in addition to whatever breaks he took.
At a slow enough pace (relative to the individual), 15-20 miles isn't a hard run for many distance runners. (For the BQers in the bunch, their recovery pace was faster than my race pace. However, their race paces would be considered recovery paces for professional marathoners.)
Does your calculation factor in the lost time when he had to stop due to immigration, war zones, being jailed for weeks? IE. When he does run, he could be running 25 miles/day but on some days, he runs 0.