I'm 55 and have had the ATB in mind since about June. I have been doing a bit over 300km a week these last few weeks as well. I've done a bit over 3,000k since June. The ATB has been a good motivator. Rest days are very important too I've learnt, in order to let your body recover, to avoid over-training or just getting plain bored.
This last week I'm planning:
Monday - 42k (Mordialloc & return from my place)
Tues - rest day
Wed - 80k Frankston (top of Olivers Hill)
Thurs - rest day
Friday - 42k (taking it steady) or more likely just cycle into Telstra Stadium and back
Sat - rest day
Sunday - the big day.
I've found Joe Friel's book 'Cycling Past 50' very encouraging and helpful with lots of advice, training routines, etc Worth buying.
Riding a 24 yr old 25" Carlton Super Course