Manhattan high school English teacher Tim Donahue, 44, was the first runner to race up the 72 floors of 4 World Trade Center Thursday.

He did it in 8 minutes and 56 seconds -- despite an injured knee.

Catching his breath seconds after crossing the finish line, Donahue said "I could have done it again." He was one of more than 700 vertical runners who raced to the top of Ground Zero's newest building to raise money for cancer research.

Running up the skyscraper's pristine wide staircase was like breaking in "an unused baseball glove," said Donahue, who has run to the top of the Willis Tower in Chicago and the Empire State Building.

But running up 4 WTC held special meaning for Donahue: "This is my home. I live here and it's pretty remarkable to be in this building that was built on something that was destroyed."

The teacher at the Birch Wathen Lenox School on the Upper East Side said he opted for the new athletic challenge six years ago when a knee injury prevented him from running horizontally.

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This is the first time that the public has had access to the building's top floor, which offers panoramic views of New York City. Silverstein Properties offered the space to the Damon Runyon Cancer Research Foundation for the charity event.

An estimated $170,000 was raised in the event, organizers said. The money will be awarded to young scientists working on cancer research.

"This is to support great minds who will break cures for cancer," said Lorraine Egan, president and CEO of the Damon Runyon Cancer Research Foundation.

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"This is such an exciting event because it goes hand and hand with the rebirth of downtown," said Egan, who hopes to see the race become a tradition.

Lydia Finley, 30, of Manhattan, who volunteered at the event and is a scientist working at Memorial Sloan-Kettering Cancer Center, said her team is studying how cancer grows, "what fuels it and makes it grow faster than normal cells. This is real promising research."

FDNY firefighter Ray Farrell, 53, of Harlem Engine Company 53, Ladder 43, and his crew of three firefighters finished the climb in less than 20 minutes, finishing among the first tier of racers.

The run, he said, "is for a good cause. There are a lot of guys on the job who are battling cancer."