Haven't played Halo, haven't read the book, but from what I know, this is my guess.
Marathon started in the Tau Ceti system. Then M2 and MI were in the Lh'owon system. I'm guessing that Earth lost contact with Tau Ceti after the AIs went rampant, and by the time they could've got a rescue team there, the fun was already happening in Lh'owon. Earth kept sending out colonies, and found the Covenant, etc., hence Halo.
Take this with a grain of salt (is that how you use that expression?), cause I haven't played Halo. I've seen the first few minutes of single player and a few trailers. This is how I link them together.
