The Marathon doesn't tend to fit many newborns well, though that varies.
I used a convertible from birth with my DD, and I'd do it again - I hate infant seats! Just a personal preference, but I completely understand wanting to skip it!
My DD was in a Roundabout, which has the same lowest-harness slots as the Marathon. Britax has changed its manual since my DD used it, and requires a better fit. While that's a good thing, it means that most newborns can't use their convertibles.
There are other convertibles available that fit newborns better, though. I realize you're trying to avoid buying another one, though, so I'm not sure that's much help. Is your Marathon near expiring? If it has a few years left, you could sell it to help cover the cost of a different convertible.
Some of the convertibles that fit newborns well are the Sunshine Kids Radian, the Evenflo Triumph Advance, the Compass True Fit, the Cosco Scenera, and I think the Recaro convertibles (not sure on those).