I respectfully disagree Rice773. 40 miles a week may very well be a nice volume for this young man, but a survey of one is not good enough to dispense advice from. 70 miles per week would be rather short for a marathoner actually. I have run way more than that many times and have never had a stress fracture. I competed against people who routinely ran 120-140 miles per week and were never hurt.
I'm not telling ws general to go out and crank 80 miles per week, but there's no magic number that will get you hurt. What usually gets you hurt are these items
1) raising volume and intensity simultaneously
2) not changing shoes in a timely fashion -most people can safely get about 500 miles from one pair of shoes
3) always running on hard surfaces
4) lack of flexibility work
5) increasing volume too rapidly
6) lack of weight training
7) not taking time off following the peak competitive schedule
8) bad biomechanics
An important skill to develop as an endurance athlete is to learn to tell the difference between the pain of exertion and the pain of injury. When you get the sharp biting pain or a chronic ache in an area close to a joint or in the middle of the shin, it might very well be the time to take time off or to cross train.
Another mistake many distance runners make is to not take advantage of therapeutic regeneration techniques such as ice bath, ice whirl pools, ice cups etc. Cool the achilles tendon, the patella tendon, and the shins down immediately following the workout and flexibility portion of your program.
Mileage done in a well planned progression is the meat of your program. You can't walk around constantly fearing injury or you may never reach your potential.