You will probably need to catch a train after the finish in the Mall, Charing Cross station is very close by, so look to park your van in south east London, say Eltham park south, free parking there, then on Marathon day catch the train from Eltham station to Blackheath station, about 10 minutes, lots of toilets etc at Blackheath start, then after you finish the marathon catch train back to Eltham station from Charing Cross, 30 mins, you won’t want to walk far after running 26 miles, good luck with your run
ps,Blackheath has parking restrictions on Sundays and on the marathon weekend, lots of local roads will be closed to traffic so I would avoid this area)