High-intensity interval training is great for building strength, speed, and VO2 max—the maximum amount of oxygen your body can utilize during high intensity exercise. And adding a few cycling sessions to an already extant running regimen has been shown to improve two-mile and 5k running performance in moderately trained runners as well as a parallel increase in running volume and intensity.
Unfortunately, I do not know of any studies that show comparable benefits of cycling to marathon performance.
Marathon performance relies less on top-end speed or VO2 max—those things that high intensity interval training most improves—than on running economy. Running economy is how much oxygen a person uses to run at a sub-maximal effort. (The lower the amount, the better.) Just because your body has the ability to use a large amount of oxygen does not mean that your body is going to use that amount of oxygen efficiently.
What is the best way to improve running economy? Running. (This helps us honor the principle of specificity in training—for a longer race like the marathon, more overall running is required than for shorter distance races.) However, strength training can also play a role. Plyometrics, short hill sprints, and other forms of strength training help runners achieve shorter contact time with the ground and improve neuromuscular fitness. (Personally, I’m a big fan of short hill sprints—ala Brad Hudson, lunges, jump roping, and eccentric calf strengthening—especially for Achilles’ tendon problems. For the core, I like bicycle, plank, and bird dog. And for the upper body, push-ups and pull-ups.)
One has to be careful here, though. High intensity strength training—like any other high-intensity exercise—carries a higher risk of injury than less intense exercising. As well, adding unnecessary bulk to your upper body can hurt race performance.
Race specific training will also help teach the body to use oxygen more efficiently at a given pace. Therefore, along with an increase in running volume, runners looking to improve their marathon performance should also look to add tempo runs and/or cruise intervals to their weekly schedule. In fact, in the past few weeks, RunningTimes.com has profiled two surprise finishers from last month’s men’s Olympic marathon trials, Jimmy Grabow and Ricky Flynn. Both men ran good marathons by increasing their overall volume and adding longer tempo runs to their schedule. In fact, Grabow eschewed intervals totally in his preparation for the trials.
Can someone perform decently in a marathon by running three times a week and cross training three or four more times during the week? Yes, of course. (Although, contrary to what Mr. Lavelle, the trainer quoted in the boston.com article, states, no evidence exists to show that swimming or Zumba will contribute directly to improved running performance. If you cross train, stick to the elliptical trainer, strength training, and/or cycling.) However, for optimal performance a runner should run as many miles as possible while including as many quality miles as possible without becoming injured. This limit will vary by person, and this is why coaching is as much an art as it is a science.
For a nice (four part) discussion on Running Economy see: http://www.sportsscientists.com/2007/12/running-economy-introduction.html
For a nice list of the benefits of higher mileage see: