I know mine's been good when my shirt is wet and I look up the hill back toward my house and ask "Run? Walk? Call a cab?" (It's only 1.5 mile) and I am leaning toward the cab ... haven't done it, though I did accept a ride from my son once when he found me trudging home.
And on one particularly ill-advised summer run, I aborted after 3/4 of my run was done and called him to pick me up. 1520 days ago