Nagato Makes Moving Easy
From Halopedia, the Halo wiki
The 3-hour time limit is calculated as the sum of the player's best times on each level on Legendary on either solo or co-op; players can view their best times on their Halo Waypoint service record.
This is by far the easiest of the Legendary speedrun achievements, due in part to the fact that the Mombasa Streets levels do not have campaign scoring and so do not contribute to the player's time. Most players will unlock the achievement on a regular playthrough without even attempting a speedrun. The game's par times total only 90 minutes; half of the time limit. At the very least, players should be able to unlock the achievement once they have completed a Classic achievement run on Uplift Reserve.
The achievement is named for Halo speedrunner HLG Nagato.