Running is pretty high impact. The high impact often produces injuries in the knees and feet. I have the notion that running "beats up" the runner. A marathon really wrings out a runner. Also you don't hear of runners running 2 marathons on days back to back.

Cycling is low impact. As a result, cycling allows the person to sustain the exercise far longer than a runner can. I figure a century (100 miles) is about 50% more effort than a marathon, but subjectively it does not "feel" like it. Sure a century is a decent ways, but I have done centuries on four consequtive days. I could have done more but things like life interfered with my cycling. I have never heard of a runner doing 4 marathons in a row (but I suppose someone has).

As for the two, does doing one help the other? Well, they both a leg sports and both are aerobically based. So yes, to a degree. But cycling uses a pretty different leg motion than running so they don't really benefit each other much. If you enjoy both there is no reason for you not to do both. I suppose that by focusing on one, you would do more to maximize your potential in the one you specialized in. But I rather doubt that any of us on this board is likely to win a stage in the Tour de France or the Boston Marathon, so we are better off doing want we like rather than fixating on the squeezing the last bit of performance out of our bodies.