Steve Blum's time during next Monday's Boston Marathon hinges on where Mother Nature is at the Hopkinton starting line.

Blum, 45, of Sharon says the weather loomed large during the two other marathons he has run.

It was 93 degrees during the 2007 Chicago Marathon. And 14 degrees during the 2008 Philadelphia Marathon. "And 14 really isn't a good day to run fast," Blum says.

For that reason, Blum says that when people ask him for his Boston expectations, "my answer to every single one of them has been consistently, 'I'll know the day before race day.'"

"Nature is what nature is going to be," Blum said today.

"You just need to do what you need to do, and just go out and enjoy it."

Blum is among 20 Sharon residents running in this year's Boston Marathon, according to the race website.

Blum said he is running with the Dana-Farber Cancer Institute team, along with residents Steve Ferris and Eric Sneider. The team has about 550 people, and has a training program, Blum said.

"It really wasn't a second thought," Blum said.

"It was the one organization that I felt really connected to."

Blum said he and his son David, 13, who attends the Sharon Middle School, collect donations for Dana-Farber after its trailer during movies at the Showcase Cinema De Lux at Patriot Place in Foxborough. Their third year doing this starts in May, he said.

"It seems like just about every week, somebody new in my network has been diagnosed with cancer," Blum said.

"This (running with the Dana-Farber team) is an opportunity to make a difference."

Blum is putting his best foot forward after starting marathons a little later than some.

"I was the slow kid," he said.

"I just avoided running, because I knew I wasn't that fast."

His wife's family running in the Falmouth Road Race found him first as a spectator there.

"I decided I'm not a very good spectator," Blum said.

Blum ran his first Falmouth race in 1994.

"I realized that there wasn't a lot of runners weren't very fast, and that was okay," he said.

"Over time, if you do it long enough, you gain speed. Whether you intend to or not, you gain speed."