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.
The Olympic 1,500m silver medalist is happy and relieved with his multi-year sponsorship contract.
The Massachusetts native will talk about training for her second Boston Marathon in the wake of last year's bombings