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.
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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.

Brig Gen Tlas, said to be in his mid-40s, is a member of Syria's most powerful Sunni family, which has given its support to the Assad's Alawite clan for decades.
The general's father, Mustafa Tlas, was Syria's longest serving defence minister, who helped to ensure Mr Assad's succession to the presidency in 2000 following the death of his father.
As a young man, Gen Tlas also attended military training with President Assad.
So, naturally, he was a member of Mr Assad's so-called inner circle, serving as commander of an elite Republican Guard unit.
The general - together with his wife - was also seen as a leading figure on the Damascus social scene, entertaining foreign envoys, artists and the media.
But sources say he has become increasingly frustrated in recent months over the violent crackdown by the security forces on protesters.
He was reportedly under a form of home arrest since May 2011, following his meeting with the opposition to try to start a political dialogue and also because of his opposition to the clampdown.
The current regime, he once reportedly said, was taking the country to hell.
Most of Syria's leaders are Alawite, but Gen Tlas is a Sunni Muslim, and his desertion may encourage other Sunni officers to consider their allegiances.

Fifty-one Syrian refugees arrived in Belfast last month under a government scheme, having been deemed as being at "real risk" in their home country.
Employment and Learning Minister Stephen Farry said language classes would help people settling in Northern Ireland to "integrate into society".
The lessons are expected to cost about Â£20,000 each year.
A second group of refugees is expected to arrive in Northern Ireland before April, and they will be settled in Londonderry.
The lessons will be provided at further education colleges, but would not apply to economic migrants.
Mr Farry said it was in the "public interest" to help refugees settling in Northern Ireland.
"The measures ensure that no one who comes to Northern Ireland seeking refuge from persecution or war is disadvantaged simply because of their immigration status," he added.
He said the lessons would be offered to all people in Northern Ireland with refugee status.
"Analysis indicates that demand for provision is relatively small so any additional costs will be affordable within the department's budget."
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.
[NEXT_CONCEPT]
Manaf Tlas, a Syrian general, has deserted the regime of President Bashar al-Assad and left the country gripped by continuing unrest.
[NEXT_CONCEPT]
Refugees in Northern Ireland will be offered free English lessons, a Stormont minister has said.