That depends on how much climbing there is and how hard you ride. 100 flat km in 8 hours isn't hard and I'd say that anyone who's not terribly unfit should be able to do it. The same thing in 5 hours isn't super easy but still doable, although you do need to be somewhat fit - I did my first one last weekend (linking just to prove I didn't make it up). Doing a metric century with 1200+ m of climbing at an average speed of 30+ km/h, takes a lot of power and endurance and is pretty much impossible to do without some serious training. And FWIW, cycling and running are two VERY different things, using different muscles in different ways. I did that ride and felt like I still could do a bit more, but all I can run is only 2 km or so. edit: One more thing. Fatties who aren't more than moderately overweight (BMI ~26-28) and are active (not lazy, just overeat) do tend to be surprisingly fast on the flats, because they're powerful (you know, from carrying all that weight around all the time) and that's where it's mostly about raw power and aerodynamics with weight having little effect. More power = more speed. That, of course, changes at the first steep-ish climb where their extra power can't compensate for the extra weight, and a less powerful but lighter rider can drop them quickly...