Blitzgiving - S6-E10
Trivia: During this episode we see a character called 'Blitz' who is played by Jorge Garcia. Jorge played the character Hurley on Lost. In the flashback of the events when Ted left the bar, Marshall asks everyone to shout out some random numbers. Jorge shouts out the numbers: 4, 8, 15, 16, 23 and 42. The exact same numbers that are a key point of Lost and the numbers Hurley played to win the lottery.
Answer: After running the marathon for Marshall with no training, he's allowed to ride the subway for free. While sat on the train, the muscles in his legs start to seize up after the marathon to the point it is painful to move his legs. This is why he can't move them. It's the same reason your body aches after a good session at the gym the day before.
Ssiscool ★