I was just gonna post about this. Big up to David Walliams! I do hope he succeeds in his quest. He said it will take about 16 hours in total. That is a lot of swimming. In training i think he said he did 8 hrs at any one time. Good luck to him!!!!!
its about 21-23 miles but most swimmers will end up swimming about 30 - 35 miles because of currents and tides.
last time I checked he'd reached the 19 mile mark so its so far so good!