Halo: Reach - They've Always Been Faster Achievement

Cleared the 2nd mission without setting foot in a drivable vehicle.

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You must do this on Normal difficulty or harder.

At the first area you find a drivable vehicle during the 2nd mission(Winter Contingency) after the first firefight with the Covenant. Avoid all vehicles during the 2nd mission. First you must find a Covenant outpost, then it will tell you to go save the survivors. After you have found and fought off the Covenant drop ships that come, you will have completed this.

This is pretty simple and easy to achieve, I recommend you play on Normal if you are going for the achievement.