Boston Marathon Qualifying Times Chart and Predictor.  An interesting take on calculators:

For me:

• (for 100 miles per week) The slowest you can run and still qualify for the Boston Marathon is 3:10:59.  This is a 7:17 per mile pace.  If you train at 100 miles per week, you need to run a 5 K in 20:20, which is 6:32 per mile pace.
• (for 40 miles per week)  The slowest you can run and still qualify for the Boston Marathon is 3:10:59.  This is a 7:17 per mile pace.  If you train at 40 miles per week, you need to run a 5 K in 19:05, which is 6:08 per mile pace.

Interestingly enough, my peak 12-week training period was about 43 mpw.