According to normative data found in the American college of Sports Medicines Guidelines for Exercise Testing and Prescription, the 50th percentile for VO2 max in 30-39 year old males is 42.4 ml/kg/min. That's probably the equivalent to a sub 7 mile (using Jack Daniel's VDOT, for instance) . While I utilized these norms in classes I teach, I don't think these "norms" are a true representation of the current US population, but perhaps a historical or ideal distribution. (there were less than 1000 people evaluated in this age-gender group for these norms, and unfit people are far less likely to volunteer for research studies like this)