Ta skydriller.
The marathon doesn't go near Earls Court, it stays in East London mainly, so you won't be trampled by thousands of sweaty grim-faced runners if you do go Sunday. London is a very big, sprawling kind of capital; also the 26.2 miles double up on themselves.
However, from the point of view of public transport, it will be busier because of the marathon. And I wouldn't even think about driving, personally (though I know how wedded some people are to their cars.
)
P