Here are the locations of the seven terminals that you need to find to unlock the "Marathon Man" achievement. For future reference, when people ask where the terminals are, please link here instead of other sites.
First three terminals: The Ark
1) When you first go inside the structure at The Ark, look to the left and you should see a wall. Go through the door and you can't miss the terminal.
2) When you press the button that makes the bridge for all of the tanks and warthogs to cross, turn around and the terminal will be there.
3) After battling the scarab, you'll head into another structure. Go through and you'll eventually get to a room with sleeping grunts; after this room is a room with two paths: a door and some stairs. Under the stairs is a door that will lead you to the terminal.
The next three terminals: The Covenant
1) The first terminal is located in the first structure you enter. When you get to the elevator with the blue light, go straight past it (you'll have to jump across) and you'll run right into the terminal.
2) When you get the hornet, fly to the first tower on the right (not the tower with all of the enemies in it). When you get close, the Arbiter should tell you that the tower is already clear, or something similar. Land on the tower and proceed inside to see a dead Elite lying on the ground; the terminal should be just near him.
3) Just like the first terminal, only this time on the third tower. When you get to the elevator with the blue light, go straight past it and you should hit the terminal.
The last terminal: Halo (last level)
This one won't take long at all. When the level starts, go straight ahead until you seem some lit-up beams going across the roof of the cave. When you see these, look to the right and you should see a small path; follow this way to the last terminal.
