One nice thing about trains is that you don't need to just sit: there is a lot of room to move about. Also, the trains you use on the Trans-Siberian route stop all the time, often for half an hour or longer at a time, so you can get off the train for some proper exercise. Still, I would not recommend doing the trip on one go: 24 hours is just about long enough stretch to spend onboard any train. There are a lot of cities worth visiting along the way.