Your regular 20km rides will set you up well. If the 2006 ride is anything to go by, BV will organise an information day with coaches to tell you about training etc. Their programme will get you increasing your distance over a period of about 3 months.
Also the WARBY's in Victoria usually organise some regular rides. These are great as they are with other riders, many of whom you will have chatted here, so you can meet up.
I'm hoping similar things will happen in WA.
The 20kg bag limit is hard for solos. If you have light weight camping gear, that will help, eg, 1.5kg tent, smallish self inflating matress, and a light sleeping bag, that can save you 1-2 kg. If you use SPD clip shoes, you can get shoes that are also good for walking, so that cuts out another heavy item, shoes are heavy. I wouldn't bother with extra nutition and drinks, they feed you well, books, SLR camera are heavy. The regulars seem to cope easily, I wish I knew their secret!
Sleepeasy means you share, but otherwise the gear they provide seems OK. But you miss mucking in with the majority, and also camping with any friends you make along the way.
I can recommend visiting and chatting here periodically, you pick up lots of tips and will get the chance to make friends before the tour starts.
The photos on the brochure are taken from the part of the 2006 tour that we will also cover in 2009. Mostly in the SW forests, spectacular, even for us Sandgropers, as doing it by bike is so much better.