I've abandoned my plan to drive up from Sydney to Cairns partly because of the sheer distance, but mostly because if I wasn't able to drive for one of the days (e.g. because the van wouldn't start, or didn't feel well enough to drive) then I'd either have to push myself to drive unpleasantly large number of kilometers per day.
Instead I'm flying to Townsville, staying there a couple of nights, and then getting the train from Townsville to Cairns (
http://www.queenslandrail.com.au/) as it's cheaper than flying directly to Cairns.