I have the same problem, right at 10 miles of constant hard riding I get the numbness on the balls of my feet. The only solution for me right now is just to stop, or slow down and relax my feet for a minute, usually be good for another 10.

Same thing used to happen when I did sprints in football and track (400m and 200m), and workouts during wrestling. Right now its the biggest problem I have, but its not really that bad, and if I'm riding in a congested area I'll probably end up stopping anyways which helps things out. Just sucks sometimes when I'm out in the country and have my momentum built up and don't want to stop but then my feet go numb...