15 miles a day is pretty tame for any long distance runner. Even without rest days. You're going to be doing 15 miles in ~3 hours. That's plenty of rest time.

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.