martin- I wouldn't bother if I were you. You are going to be very busy workwise, and finding the time as you say will be difficult. Coupled with family time and the various bugs doing the rounds, could make you run down and impact on training in New Year.
4h30 in December will have no impact on your Comrades time. 4h30 in March/April will. As long as you are maintaining a level of training that keeps you fit, and mentally encourages you, then leave it for the New Year. Build up through Jan/Feb and aim to put a solid block of 10 weeks in Mar- mid May before taper.
Please feel free to ignore the above 
Gutted I am officially not running Comrades next year. I have entered the KLM competition so who knows!
Looking forward to enjoying your journies as training ramps up.