Somewhat depends on the person. I've done plenty of both, but find a century much easier than a marathon. But I work harder at a good time in the marathon than I do a solo weekend century done more leisurely. My daughter is a (barely) nationally-ranked sub-3-hour marathoner who finds even 30-40 mile bike rides daunting. I suspect if Joe Six Pack started from scratch, he'd be prepared for a century WAY before a marathon.