"Bam! Bam! Bam!" That's the sickening thud Becca Pizzi says runners make when they're out training for the Boston Marathon and bite the dust.

Or, more specifically, the snow.

"I was running on Heartbreak Hill and people were hitting the ground so hard they couldn't get up. You hear those 'bams!' and you're afraid they've broken something," said Pizzi, 34, a day care center owner who's struggling to get in proper shape for her 15th Boston.

Running 26.2 miles requires endurance, but 8½ feet of snow and lots of treacherous black ice are testing this year's participants in frustrating new ways. With race day less than two months away, the relentless winter is sending some runners indoors to basement treadmills and health clubs — and driving others just plain nuts.

One of the blizzards that hit the city in rapid succession forced the Boston Athletic Association, which administers America's premier marathon, to cancel a training clinic. Though the worst of the winter now seems past, there are only 48 days left until April 20, the 119th running of the venerable race.

That's not a lot of time to get legs and lungs in shape. And many streets and sidewalks remain slick, making it difficult and dangerous to share narrower-than-usual roads with drivers and log the 20-mile runs that are a staple of marathon training.

"I'm so sick of sliding around on the snow and ice," said Peter Horning, 30, a chemist who lives in Boston's Chinatown. "It's way worse than running on beach sand. It's exhausting. Every three steps count for seven."

Thirty thousand runners from 90 countries and all 50 states will be represented, including more than 4,800 from hard-hit Massachusetts alone.

Motivated by a desire to run a personal best or to raise money for charity, they're doing what they can to brave the elements or find creative training alternatives.

Some, like Eric Bergen of the Greater Boston Track Club, have resorted to doing all their running indoors on a track or a treadmill. "I haven't stepped outside for a single run in five weeks," he grumbled.

Many marathoners despise what are often called "dreadmills." Their advantages are obvious: There's no weather or traffic to contend with; runners can go as fast as they want, with surer footing; and sweaty indoor workouts can be good preparation if race day is unseasonably warm.

So, too, are the disadvantages: running mindlessly in place, mile after mile for up to three hours at a time, on a bouncy belt that doesn't mimic Boston's potholed asphalt or the course's challenging hills.

Despite the brutal conditions, Kevin Cordaro of Whitman, a cardiology researcher whose Boston goal is 2 hours 45 minutes, managed to run 80 miles last week. But 56 of those were run outdoors, at night, on a half-mile loop in his neighborhood to avoid cars and plows.

That's 112 laps — only slightly less mind-numbing than running in place or circling a track.

"Depending on how many street lights are on, the scenery ranges from stagnant to nonexistent," said Cordaro, 25, so bored he's been listening to "This American Life" and other podcasts while putting in his miles.

To avoid falling outside, others wear cross-country spikes or snap-on devices designed to provide traction.

Gary Circosta learned the hard way how lethal winter can be.

Shortly after Thanksgiving, the 67-year-old dentist — training for his 16th Boston Marathon — fell hard on black ice near his home in central Massachusetts. Two broken ribs punctured and deflated a lung; another rib tore a hole in his diaphragm. He wound up in emergency surgery for septic shock and almost didn't survive.

"I have never run less," said Circosta, who's still determined to make it to the starting line in Hopkinton.

With marathoning as much about mental toughness as physical fitness, runners like Anthony Walsh of Dublin, Ireland, aren't just enduring the snow and cold — they're embracing it in full Beast Mode.

Last weekend, the 31-year-old Harvard Medical School cancer researcher ran 24 miles on the Boston course in shorts and a T-shirt.

"Having the snow to fight against has been motivating," said Walsh, who's gunning to break his 2:36 personal best.

"Boston people are incredibly encouraging. They've been almost universally supportive," he said. "The worst they'll shout is, 'You're crazy,' but they say it with a smile. Bostonians understand running, even if they're not runners."