OH my God this makes perfect sense!
Chelsea Bridge
"Seven steps down, in a building, I found three rooms. Each room contained a special key to a special lock. Past the lock, I found a box with a story inside. A wonderful story. It was a story I had seen before, but it felt . . . strange."
Seven steps down = 7 years of Marathon
3 Rooms = 3 marathons...each contain a "key" (part)
A story I had seen before but it felt strange = The Halo story..hence the Marathon symbol in the middle of the Halo logo
DOH! there's no doubt in my mind that that's what the reward is or something very close to that!
