Taking part in a charity static cycling thing with work, and the 850 or so of us doing this have cyclewd once round the world (within 10 hours) and well on our way to the double-circulation within 24.
Doing it in teams of six, and taking shifts, so we each ride 4 hours - I've shared the 2-4pm and 8-10pm shifts thus far; got 2-4am and 8-10am to get through. Strangely my mileage is going up with each 30 minute stint.
I actually cycled home after 4pm so I could get seomthing proper to eat at home (as well as fetch some things for overnight). Harder work on the exercise bikes than it was riding home and back in!