Yea, the human times weren't bad, but the winner of Will Hill Man wouldn't finish in the top 20 in a large marathon. The difference in time between first and 20+ place would be enough to overtake the horse I believe.

"I also think that the horse opposition was just decent local horses."

I'm not sure how you came to this conclusion, but the horse that won last year, Druimghigha Shemal, had been trained specifically for distance over steep, hilly countryside. Past that, I've never heard of an elite marathon distance horse. But it's a big strange world out there.

"As it is, I think the William Hill event shows that the horses are faster but not by all that much."

On a cross country course with non-professional marathon runners, yes. But like I said above, I'd bet on an elite runner over an actual marathon course.