In short, in life, as in a foot-ball game, the principle to follow is:
Hit the line hard; don’t foul and don’t shirk, but hit the line hard!

Theodore Roosevelt (1858-1919) American politician, statesman, conservationist, writer, US President (1901-1909)

“What We Can Expect of the American Boy,” St. Nicholas Magazine (1900-05)
    (Source)

Reprinted as "The American Boy" in Roosevelt, The Strenuous Life (1900).

Roosevelt used this general phrasing on multiple occasions, so various forms can be found attributed or associated to him, such as:

• "In life, as in a football game, the principle to follow is: Never flinch. Never foul. Hit the line hard."
  [Source]
• "Don't flinch. Don't foul. Hit the line hard."
  [Puck]
• "Don't fowl, don't shirk, and hit the line hard!"
  [Autograph]
• "Don't flinch, don't fowl, and hit the line hard."
  [Speech (1913-07-03)]