Blower fans draw more current than most folks suspect. Newer MB cars have a dedicated 40 amp circuit for powering the blower motor, and that does not even include the control circuitry.

A quick google shows the Fit has a 40 amp-hour battery. That means it can supply 1 amp for 40 hours. However the inverse is not true, it cannot supply 40amps for 1 hour. At higher current draw rates the battery discharges much more quickly.

Given the blower fan and radio perhaps you were drawing 20 amps for the better part of an hour. While that certainly didn't flatten the battery, it could well have discharged it 50% or more. That's likely enough to prevent the car from starting.

BTW, the ginormous group 49 batteries used by MB are 100 amp-hours. You would have been fine.