How far is it from the Arc de triomphe to La Défense?

Not too far; can be done by foot. Approximately 1.5 miles and downhill from the Arc to the Louvre. Easier by subway, two station stops. You walk down the Champs Elysee, past the Napoleon bridge to the right and the Grande & Petite Palaces, past the obelisk, through the Orangerie and the Garden of Tuilleries and there is the Louvre.