Continuity mistake: During the scene back in college, when Marshall and Lily are listening to Dr. X's show, at the first shot of the radio, you can see some popcorn there already, although Marshall has not thrown any, yet. Even if you claim he's been throwing constantly, a few shots later they vanish, and reappear.
Other mistake: At the start of the episode it can be seen that the DVD player only has a power cord connected. Shortly afterwards only one cable (power) is seen connected to the TV set, then a few seconds later Ted plays a DVD.
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 ★