I don't know why, but I've always felt biking up a long hill was more painful than running it. If I get so gassed riding up a steep hill, I have no philosophical problem with walking/jogging it, since I'm convinced there are gears below which it is easier to walk the bike than sustain a balanced spin. My theory is that the 20lbs of bike becomes significantly more of an issue then any gearing advantage you can create.

But even when you can sustain a decent pace, I feel the pain greater on a bike than when I ran regularly. And I've run the heartbreak hills during the Boston marathon several times.