Mayer, in red, with Chef Gary Danko (middle), and 100 Mile Month champs Photo by Googler Wesley Chan

Last Monday evening, in the backyard of her Silicon Valley home, Marissa Mayer stood before a crowd of 200 fellow Googlers and their significant others, fed them roast quail and herb-crusted roast bison loin, and feted them for going mobile.

"We walked more than once around the earth at the equator—or 16.7 times around the moon," Mayer declared at the award celebration for the fourth annual "100 Mile Month Challenge."

This is a physical fitness contest that Mayer, who is VP of Consumer Products and oversees Google's (GOOG) local and mobile products, created to get her colleagues away from their computers and walking outdoors. Mayer came up with the idea, she says, during her own personal fitness kick, when she reviewed her spreadsheet (tracking miles she runs daily) and realized that by running just 10% more, she could do 100 miles per month.

Not satisfied with setting the goal only for herself, she challenged the people who report up to her to strive to do 100 miles monthly too. Mayer's fitness crusade seems to be working. This year, a record 348 people--Googlers and spouses and other partners--signed up to compete. From July 15 to August 15, they wore Omron pedometers and tracked their miles walking, running, cycling (five miles on a bike counts as three miles walked) and swimming (a mile in the pool equals four on foot). Since a typical office worker walks two miles daily, Mayer says, the rules are that contestants must subtract two miles daily from their monthly total. Nonetheless, overall performance in this year's Challenge was impressive: 220 contestants reached the 100-mile goal.

In a corporate culture where spreadsheets are sexy and nothing is cool 'til it's quantified, the contest stokes serious competitive juices. Last year, Laura Melahn, in marketing at Google, captured the "Biggest Day" award by trouncing two male APMs (associate product managers) who had each logged more than 40 miles on the final Sunday of the competition. Melahn's "Hail Mary" on Monday was to do everything humanly possible while also doing her job: Starting at 5 a.m., Melahn biked to Google, answered emails on the treadmill in the company gym, swam in the Google treadpool, and walked/ran every minute she could. Her Monday total: 58.78 miles. Turns out, that's nothing compared to what Dale Hawkins, an engineer who works on Google Maps in Boulder, Colorado, did this year. In one day, while running the Grand Mesa 100 trail race, he did 65 miles.

Hawkins took home this year's "Biggest Day" award. Another engineer and runner, Craig Robinson, won "Most Total Miles" by logging 302.41 miles in a month. His prize last Monday night was two board games, Risk and Monopoly, "because he took some risk to get a monopoly on the miles," notes Mayer. She takes care to celebrate ordinary exercisers as well as the superjocks. Last Monday night's prizes included a "Skin of the Teeth" award for the Googler who reached 100 miles, just barely. She also cast the spotlight on contestants who lost a lot of weight.

Photo by Googler Wesley Chan

As for Mayer, she nearly failed in her own competition. After running the San Francisco Half Marathon on July 31, she got busy with work and travel and fell woefully behind in this year's 100 Mile Month Challenge. The weekend before the contest ended, she flew to Wisconsin to visit her parents, figuring that traversing airports would add at least a couple of miles. Hardly. ("Airports are deceiving. You think you walk more than you do.") That Sunday in Wisconsin, Mayer walked with her dad and ran on a treadmill. She did 13.4 miles that Sunday, just enough to earn her an invitation to her own party.

Mayer, 36, was Google's first female engineer and is the youngest person ever to make the 2010 Fortune Most Powerful Women list. Her colleague Susan Wojicki, who oversees Google's ad products and engineering and more, is on Fortune's just-released list of the Smartest People in Tech. Wojcicki was No. 43 on last year's MPW list, and Mayer was No. 42. Where do you think we should rank Wojcicki and Mayer on this year's MPW list, to be revealed in a month?