Here's what I've discovered online, as it pertains to Marathon inside Halo.
1- One of the "armor permutations" is the "Marathon Helmet", which you can check from the URL below. From what I've heard, it kinda looks like a bug's face when playing.
2- There are quite a few hidden terminals throughout the game. 2D, text-based information that, from what I've heard, comes from the Forerunner. I don't know the information; wouldn't be surprised of some Marathon-related stuff.
3- Getting all of those terminals unlocks the Marathon Man achievement, the highest rated achievement in the game.
I realize this has prolly been said, but oh well.