It’s almost time for the Boston Marathon.

And if you’re familiar with the Boston Marathon, you’re probably also familiar with the course and its reputation. Boston starts with a series of terrifying descents that shred a runner’s quads, followed by a series of grueling climbs. The most notorious of these hills is the famed “Heartbreak Hill.”

But, as a Boston local and multi-time Boston Marathon finisher, I believe the worst of these grueling climbs is actually a small uphill through the Commonwealth Avenue underpass below Massachusetts Avenue during the final mile. You read that right: this is the toughest hill—even worse than Heartbreak. Though it gains only a handful of feet in elevation gain, it makes up for that by being the cruelest.

It’s understandable that many see Heartbreak Hill as the worst hill. It’s at mile 20, which is already a particularly challenging point in any marathon. And it’s a sharp incline with about 91 feet in elevation gain.…