A man goes to his doctor to complain about his lack of stamina in bed, when he makes love to his wife.
"Obviously, you're simply out of shape," the doctor ascertains. "You need strenuous excercise to regain your youthful stamina. I want you to run 10 miles a day."
A week later the man phones the doctor and says,
"Okay, I'm 70 miles outside of the city; what do I do next?"