I'd say the sub 6 hrs is harder because there's more variables involved after 40 miles than just leg speed. I've continued to train like a marathoner. I don't believe you really need to adjust your training unless there's specificity to the terrain (hills/mountains, technicality). You have to learn how to take in more fuel and hydration, keep a level head to persevere, troubleshoot anything, and then after 35-40 miles you find out how good of a fat burner you are.