Two explosions near the finishing line killed three people and injured at least 140 about two hours after the winners had completed the race.
"If this had happened two hours earlier, maybe I would have been among the victims," Mr Korir said.
He had been celebrating Kenyan Rita Jeptoo's victory in the women's race.
"The joy that we had has all been taken away," Mr Korir told the BBC's Newsday programme.
Ms Jeptoo comes from the Cherangany constituency, for which Mr Korir was elected an MP in last month's election.
Despite the blasts and his political career, he says he will continue to compete in marathons - even in Boston, if it is held again next year.
"My attitude to marathons will never change but my attitude to life may change," he said.
Bostonians recall 9/11 spirit
Obama statement
How secure are marathons?
He said it was "scary" when he heard the news of the blasts.
He said on hearing the news, he immediately checked on his parents-in-law, who were visiting, along with his coach.
Kenya's ambassador to the US has told the BBC that none of the eight Kenyan athletes running the marathon were wounded in the blasts.
Two South Africans were treated in hospital for minor injuries but have now been released, officials say.
The men's race was won by Ethiopia's Lelisa Desisa, ahead of Kenya's Micah Kogo.
The annual Boston Marathon this year had a field of about 23,000 runners and was watched by hundreds of thousands of spectators.
It is held on Patriots' Day, a Massachusetts state holiday which commemorates the first battles of the American Revolution in 1775.
UK police are reviewing security plans for Sunday's London Marathon, the next major international marathon, following events in Boston.
Kenya's winner of the 2012 Boston Marathon, Wesley Korir, who came fifth this year, has told the BBC of his fear after hearing of the deadly blasts.