Let's see. Years ago in Backpacker Magazine it was asked what is the maximum reasonable pace for packing (with full pack). Their answer was 5 MPH. That translates to a marathon of about 5 1/4 hours.
But would you really call this running a marathon? That is the big problem. There is a line between running and walking, not so in cycling.
Also a lot of marathons are rather flat. A hilly century is more difficult by any measure. But what is considered finishing a flat century is so easy it can be done by someone who has been of the bike for a long long time.