I've installed a Britax Wizard RF in my Nissan Versa (2007 S Hatchback.) I think the shell is identical (or at least very similar) to the Marathon's.
It will fit best on the passenger side, though you will find a Versa to have SIGNIFICANTLY more rear passenger room than a Honda Fit.
I was able to wrap the rear-facing tether anchorage strap around the front passenger seat post for tethering RF.
I test drove both the Fit and the Versa when I was car shopping in 2007. I opted for the Versa in large part because the back seat is so roomy for a compact car. I've had three child seats across installed in my Versa, which wouldn't work in a Fit with any seats I've seen. I drive three kids (Britax Parkway SG booster; RideSafer Travel Vest; SK Radian80) to school every morning in my little car...
...but I'm shopping for a minivan right now because it has gotten too close for comfort!
If our trip to school were shorter so carpool wasn't a necessity for my mental health, I would drive my Versa 'til its bitter end. I'm very, very happy with the car, but it is FULL with three kids, two backpacks, my huge "Mommy bag", my emergency supplies duffle bag in the back, and a spare seat (Britax Regent) strapped in the front seat because one time another desperate parent begged me to drive her 12 yo kid home even though he had to sit in the front seat, and I did it and still hate myself for not putting the big kid in back and re-installing my harnessed toddler up front...
--willo