Yes, the ADC wants a source impedance of <=10K. Yes, it is more convenient to use a resistive divider that has an output impedence of >10K. Solve the problem by shunting the ADC input to ground with a 0.1 uF capacitor, after all, the clouds do not move that fast...
I have used this trick when sensing 12V battery voltage, where I wanted the two resistors in the divider to be high enough not to provide much leakage to the battery (>~100K). The capacitor holds the voltage at the tap constant while the AD does its dance ( a few tens of micro-sec ). The DC input leakage spec for the AD is ~1uA, which does not load the voltage divider much. Again, the battery voltage changes slooowly.
Besides, you are supposed to use an "anti-aliasing" filter upstream of any ADC input...