I honestly don't know if 20k is theoretically doable. 19k took a while and a lucky run which probably should have been about 18.6k given the pace... insanely lucky ending! Although at a glance 19k looks very solid I think you might be able to get the start much quicker than I did. Maybe I'm being silly for even considering 20k here - but calling something impossible doesn't sit right with me any more ;)