The more interesting question is how to best utilize those 10 miles. 3 5k efforts a week is likely the best type of option. What if it were the 10k? Seems like in that case 2 hard 5-milers a week is optimal. In the case of the marathon, you'd definitely have to go to one hard 10 a week.
This really begs the question of optimal mileage in general. Clearly at some point there are diminishing returns, and optimization depends a ton on the event you're training for obviously. I think a lot more mid d and distance guys could be successful off 30-60mpw with cross training than the 70-120 most of them do.