The 59-year-old will run his race in memory of a child killed in last year’s terrorist attack.
Flanagan wants to win the Boston Marathon more than any other race.
Webb, 31, retired from track and field this past winter.
Kastor runs 1:11:38 to set new U.S. 40-and-older mark.
Mo Farah placed eighth in his full marathon debut but vowed to run another one.
Vail's finish is the highest by an American in London since 2009.
Tibo Vegh, Michelle Renee Langan won the half marathon races at the inaugural Raleigh event.
The American said his run-up to London was the best training for a marathon he's had.
The two runners will take to the streets of London in Mo Farah’s shadow this weekend.
Sunday's London Marathon features four of the 10 fastest marathoners in history.