A new Marathon: Infinity scenario has been released. Return To Marathon will indeed return you to Marathon:

Revisit the Marathon, 184 years after you saw it last. Explore familiar areas, but beware new dangers! Things have gone horribly, horribly wrong after UESC abandonment... and it's up to you to sort them out.

Wander through three enormous levels - large parts will be familiar, some parts will not. Remember that this is a deserted, derelict moon - there are pitfalls and hazards at every turn. The obsolete air recycling system has begun to break down without maintenance... there are times you'll be scrambling for an oxygen recharger. Things have taken up residence in once-pristine hallways - and very, very few of them are friendly.

Check out Return To Marathon and download chapter 1.