At The Frontal Cortex Jonah Lehrer has a post on marathon running and memory. Stress, like the stress from running for 4 hours, is known to disrupt memory -- but all memory isn't disrupted equally. The study found that after a marathon, runners had reduced "explicit" memory (ability to remember specific words, facts, numbers, etc.) but improved "implicit" memory (the ability to remember actions, motions, processes, etc.).
Fascinating! As someone who has run -- oh, is it 5? -- marathons I definitely felt a deterioration in my cognitive abilities as the races progressed. My ability to do simple math (like calculating mile splits) withered away. But it wasn't because I couldn't add anymore -- I could do that just fine. It was always because I couldn't remember what my watch said one mile earlier. That's a distinctly "explicit" memory function.