I’ll second Zeke. To me, a marathon pace run of under 10 miles, especially as the goal race nears, should feel like a walk in the park. From about 10-15, it starts getting difficult but is still quite manageable. Over 15 starts getting difficult. I wouldn’t think of doing over 20 because it becomes too close to a racing situation at that distance. Of course, these ranges are just rough estimates but I would be quite concerned if 3-4 at my planned marathon pace didn’t feel very easy.