Last Forever: Part Two - S9-E24
Trivia: On their wedding day, Tracy shows up at McClaren's to meet up with Ted and the gang. While sitting at their usual table, Ted and Tracy kiss. Right when that happens, we quickly see Marshall hand Lily some money. This is a callback to a bet they made in season 7, when Lily bets Marshall that Ted and Robin don't end up together.
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 ★