The 2011 King George and Gold Cup winner, ridden by amateur jockey Sam Waley-Cohen for trainer Nicky Henderson, just got the better of outsider Captain Chris to win by a neck on stamina-sapping ground following heavy rain.
The 15-8 favourite, second last year to now-retired Kauto Star, was headed by Captain Chris (16-1) at the final fence of the three-mile race but rallied in the final strides to edge home.
"He was so brave. That was racing at its best. It was an honour and a privilege to ride," Waley-Cohen told Channel Four television.
Long Run will attempt to regain his Cheltenham Gold Cup crown in March after finishing third this year.