Well, there is such a thing as the Marine Corp Marathon. And UPS or Fedex trucks are often used for gear drop-off and transport to finish areas at large-scale (= tens of thousands of runners and walkers) running events. And those receiving the dropped off stuff are often/usually volunteers.
How much of the

actually happened is another matter. It's credible up to the point that a (probably many!) runner dropping off gear thanked a Marine for his or her service. The event celebrates the USMC. But after that, nopers.
BTW, a marathon runner being a "muscle bro" is dubious to me (and I've seen many marathon and ultra-marathon runners). I would not say it's impossible that a marathon runner is also a weightlifter or bodybuilder (I've known at least one), just that runners of such longer distances tend to specialize in the kind of fitness suited to that activity.