What's the normal tread wear on a pair of shoes? how many miles before you lose tread, and where should it get worn down first and by how much? just curious because this gives a good clue to what kind of shoes you should get. mine wears out fast on the balls of my toes and just outside the back of my heel. what sort of shoes should i look for if i want something as light as possible that will hold up through 70 mile weeks?