Well, I'm also someone who tends to sign up for endurance events, but without enough time/discipline to train properly. I generally can only do decent-length run/bike/swims once per weekend, and even so for no longer than 3 hours.

If they're both decent runners, I'd predict they could do a century if they got in a couple of 40 mi rides, a few weeks before the event. They would definitely be sore afterwards, but they would make it OK. Also, the 40 milers should expose any critical bike setup problems that they should address before the event.

That was exactly my level of preparation (2 40 mile rides) heading into the two-day, 150 miler charity ride I did this summer.