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From the NY Times in 2009:

An important 2008 study, this one from Stanford University, followed middle-aged, longtime distance runners (not necessarily marathoners) for nearly 20 years, beginning in 1984, when most were in their 50s or 60s. At that time, 6.7 percent of the runners had creaky, mildly arthritic knees, while none of an age-matched control group did. After 20 years, however, the runners' knees were healthier; only 20 percent showed arthritic changes, versus 32 percent of the control group's knees. Barely 2 percent of the runners' knees were severely arthritic, while almost 10 percent of the control group's were.

The science makes sense: basically, that your knees use cartilage to protect, and stressing the joint slightly simply results in the build-up of more cartilage.