About 30,000 runners will invade Washington Oct. 28 for the 2012 Marine Corps Marathon. The race's popularity has skyrocketed since its inception in 1976, when 1,175 participants undertook the lung-busting challenge.

Nationwide, people are flocking to marathons: Between 2000 and 2009, the number of participants has increased 58 percent. This growing obsession with covering vast distances by foot has me thinking about its environmental efficiency compared to other modes of transit. How green are our two legs?

Calculating the average runner's greenhouse gas emissions for a 26.2-mile distance is a fairly straightforward task.

The first step is to determine how much energy is burned by the typical marathoner. Exercise physiologists estimate that an ordinary runner weighing 150 pounds uses approximately 3,000 calories on the marathon course. The next step is to determine how many carbon dioxide equivalents are released into the atmosphere in the creation of those calories.

You see, the carbon dioxide that a runner exhales does not directly contribute to climate change, because it's a part of a natural carbon cycle. Humans exhale carbon, plants inhale it, humans eat it and exhale it some more. However, our agricultural system uses fossil fuels to produce food, which means that carbon that had been safely stored in the ground is extracted and added to the atmosphere. Growing food on an American farm emits something like 0.0064 pounds of carbon dioxide equivalents for every calorie produced. (A carbon dioxide equivalent includes emission of both carbon dioxide and other greenhouse gases, such as methane and nitrous oxide.) That means a runner eating a standard American diet is responsible for the emission of 19.2 pounds of carbon dioxide during a marathon.

A few footnotes are in order. Statisticians include all the food that farmers produce when calculating greenhouse gas emissions per calorie. If we didn't throw away as much food as we do, exercising would be less carbon-intensive by this standard.

The efficiency of food distribution and production is out of your control, but you can change such things as your diet. A calorie from fruit and vegetables is responsible for about 40 percent less greenhouse gas than a meat or dairy calorie, so vegan runners do substantially better in this department than omnivores.

You could also slow down. While slower runners get less glory, the tortoises are a little bit greener than the hares. Running a marathon at a pace of 10 minutes per mile releases about 5 percent fewer carbon dioxide equivalents than running at six minutes per mile.

What if you ditched your running sneakers and hopped on a bicycle? Just like running, it depends somewhat on how fast you go, but overall, cyclists produce far less greenhouse gas than runners over the same distance. Pedaling at a moderate 13 mph, a 150-pound bicyclist would burn around 1,150 calories over 26.2 miles. Eating a standard American diet, he would be responsible for 7.4 pounds of carbon dioxide - 60 percent less than the runner generated.

You might be surprised at how these totals compare to driving. The EPA estimates that the average vehicle on American roads emits 0.87 pounds of carbon dioxide per mile. So, over 26.2 miles, a car would account for just 22.8 pounds of greenhouse gas - about 19 percent more than the runner.

Of course, it's not entirely fair to suggest that your sedan is only slightly less environmentally friendly than your all-natural body. The carbon footprint of a car is more significant than the gas that it burns. When you take into account raw materials, manufacturing and disposal, in addition to fuel, a car is responsible for about 1.1 pounds of carbon dioxide equivalents per mile, meaning that it's true marathon footprint is closer to 28.8 pounds, or 50 percent more than running.

Some analysts have pointed out that even that estimate is low, as nationwide infrastructure required to support automobile transport - including road-building and maintenance, gas stations, etc. - is difficult to calculate.

Bicycles, too, have impacts beyond the food required by their riders. Manufacturing the average bicycle accounts for approximately 530 pounds of carbon dioxide. It's difficult to apportion that total on a per-mile basis, though. Some people keep the same bicycle for an adult lifetime, while others change every time the color goes out of style. In addition, if you use a bicycle for commuting in addition to leisure riding, the carbon you save by not driving quickly pays for the manufacturing emissions.

By riding about 400 miles instead of driving, a bike pays for itself, environmentally speaking.

And running shoes, too, have environmental impacts, although they don't compare to those of a car or bicycle. A conventional leather sneaker is responsible for 16.6 pounds of carbon-dioxide equivalents after taking into account manufacture, transport and disposal. Many marathon runners change shoes every 500 miles, which means the shoe itself would be responsible for an additional 0.9 pounds of greenhouse gas during a marathon run.