You add luggage and passengers. Boeing did it when it tested the 787 going around the world during December 2011 with its ZA006. The round the world flight had a crew of twelve pilots and engineers. It took a total time of 42 hours and 27 minutes including the two hour refuel stop at Dhaka Bangladesh.
Boeing demonstrated it could fly for twenty hours and 10,337 miles on one 787-8 configured aircraft having 12 people on-board.
Remember it was 2011 before the next improved GE or Rolls were hung on its 787 wings. It was long before the first 787-9 dropped in and don't forget the proposed 777-8X long range flyer was only a pipe dream at that time.
Qantas has thrown down the gauntlet in front of Boeing and Airbus who are expected to come up with a 20 hr flying time solution with its 7** and A350 offering. Boeing responded with a "we are working on it" response. Airbus is also doing the same, but without any public proclamation.
Wikipedia: Boeing was also studying an ultra long-range replacement for the 777-200LR, conceptually dubbed the 777-8LX, which would share the −9X's fuel capacity and gross weight. Its range will be 9,480 nmi (10,910 mi; 17,560 km) compared to 9,395 nmi (10,812 mi; 17,400 km) for the −200LR.
However, there are other quotes for the 777X's range of 8,960 miles stated for the 777-8X and the 777-9X comes in within the 7,550 mile range.
The Qantas Challenge:
A fuel sipping flight may not fly at mach .85 as that translates into 566 knots per hour. However, in an expected 20 hr flight with 300 passengers on board,the fuel sipping could be achieved traveling around mach .80 range or 534 knots per hour. Once again Boeing will look at tweaking a 777-8X engine performance using lean burn techniques with optimal engine output for a twenty hour flight. The Mach .80 version could or should go 10,068 nautical miles in twenty hours.
The 777-8X is listed with having a 365 passenger capability, but a Qantas super long range type from Boeing may only require around 300 seats thus diminishing its take-off weight significantly as any formula constraining distance includes fuel weight and passenger weights determining how long and how far an aircraft can fly.
Having Airbus competing in the Qantas throw-down for a twenty hour flyer, Boeing may not have to sacrifice Jet engine performance where GE may already have a rabbit to pull out of its engine development hat. It could propose a tweaked GE9X engine as a one-off specialty engine offered as a Boeing package to Qantas for the long haul it proposes.
What does Boeing have to over come? Airbus punching out another ULR A-350 for one. Going back to the Boeing, it starts with engine, folding wings extensions and then its own off-the-shelf technology for making a ready to deliver a 777QX passing within the Boeing assembly line. It can fly 20 hours as the 787 demonstrated in December 2011. Now it must find a way for flying 20 hrs with its passengers, additional fuel and luggage.