Runners tend to lean into hyperbole when it comes to training challenges.
To run a gauntlet is traditionally a kind of punishment or public trial.
In modern slang, it usually means tackling a challenge in an overly gruelling way, adding an extra level of demand to something that is already tough enough on its own.
For example, I could have tonight just gone out into the neighbourhood, ten degrees below zero in light snow and wind, and run a lap around the main road loop.
Instead, I went out into the neighbourhood, ten degrees below zero in light snow and wind, and ran a lap around EVERY road along the main road loop, in and out of every side street, rounding out every cul-de-sac, and weaving along every meandering turn I could find.
A three kilometre run turned into a run three times that distance, with numb toes to boot.
Maybe not quite a gauntlet, but a heckuva challenge for a late-winter evening.