The mile markers came and went with encouraging ease. It was a cool morning in Rochester for the Scheels Med City Marathon and, minus a pre-race biffie mishap, all signs pointed toward a PR.

Snaking my way through the pack, I was plotting our family vacation to Boston in April 2020. Because, the way things were going, I was a shoo-in for running’s holy grail. I’d need a time in the neighborhood of 3 hours and 5 minutes, nine minutes faster than my previous best. But, man, you should’ve seen me!

The miles were purposefully slow at first. This is smart, I thought, conserve energy. I tore through my only two packets of GU, an energy gel, early on, primarily because I was sick of holding them. Too sticky. No matter, I was cruising.

My goal was 3:10, a 7:15-per-mile pace. I reached the midway point in 1:34:41, or 7:14 per mile. Perhaps buoyed by my GU binge, it felt effortless. Doubly so after making the questionable decision to — and this is the most delicate way I can put this — relieve myself.

Without leaving the course.

Runners don’t make Boston by stopping for bathroom breaks. I’d waited too long before the race’s start to get in line for the porta potties. Eventually, I had to bail and go run. I knew it was a risk. Soon, my wife did, too.

As I approached halfway, where I expected to catch a glimpse of Marisa and our 14-month-old son, Lincoln, I was on cloud nine. So when I spotted them, I frantically pointed to my groinal region — with both index fingers — which told the story pretty darn well. For some reason, I wanted to share with my wife the excitement of pee. She reluctantly waved back. It was a special moment.

From there, things improved dramatically. Temporarily. Over the next seven miles, my 7:14 pace dropped to 6:59. If I could keep that up, Boston was a real possibility. Turns out, I could not keep it up. That is because marathon organizers keep making their events 26.2 miles. Something closer to 22 miles would suffice, no? What happens after that can hardly be considered running, at least not for me. It’s a glorified walk. When I neared the finish line May 26 in Rochester, my wife says I looked green. I had just covered the final 6.2 miles at an 8:48 pace.

There was no 3:05 that day, no Boston Marathon-qualifying standard and no PR. I finished in 3:17:19, my happy pee memories having faded into the past.

In distance-running parlance, I had hit the wall. Well, to be more accurate, I slammed into the thing and never recovered. Nothing I did worked. I was spent, cashed, a goner. It’s a scenario that’ll play out repeatedly Saturday morning along the North Shore as a field of 8,571 entrants negotiate the 43rd annual Grandma’s Marathon. Another 9,237 are entered in the Garry Bjorklund Half Marathon.

What, exactly, does it mean to “hit the wall?”

“Hitting a wall basically is when your body runs out of glycogen or gets very low on glycogen,” said former Duluth Denfeld cross country and track coach Gary Lepisto, who continues to train numerous runners. “The point at which your body starts to burn more fat than glycogen, that switchover is hitting the wall.”

That typically occurs right around two hours of running, Lepisto says, and thus one strategy he noted is to start taking GU at 90 minutes or 1:45. This, in addition to regular consumption of a sports drink throughout, has the effect of adding glycogen into your system and delaying its depletion. Your brain, which is constantly monitoring your well-being, then becomes more willing to allow you to run a little bit longer or a little bit faster.

Lepisto also says it’s beneficial to begin carbo-loading about 10 days beforehand — not simply the night before.

Runners’ best bet, though, is to prepare diligently.

“If you back up a few months, the best way to avoid hitting a wall is to be doing plenty of long runs, so you run through that switchover many times in training,” Lepisto said.

He never said if an in-race tinkle affects those glycogen stores.