MARATHON, Fla. >> What would you do if you were running across a bridge and a sheriff’s deputy, exiting from a large vehicle, ordered you to stop? For Peggy Paino, who recently found herself in that unpleasant position during the Seven Mile Bridge Run in Marathon, Florida, the response was pure criminal instinct: Run away. Only it was more of a slow jog.

“I pleaded with him to let me finish the race,” Paino, 66, said of her emotional encounter with the deputy, Frank Westerband of the Monroe County Sheriff’s Office, about three-quarters of the way across the bridge. Sorry, not sorry, he responded, essentially.

“He said, ‘Nope, you’ve got to get on the bus,’” she recalled.

It turns out sometimes you can’t outrun the law, not when the law arrives backed up by a large yellow school bus and a mandate to round up every laggard and straggler in its path.

The race takes place every spring, when 1,500 participants run across the two-lane Seven Mile Bridge about halfway along the Florida Keys. Because the bridge, part of the Overseas Highway, as Route 1 is called down here, is the only land route connecting the north and south keys, there is no flexibility in timing. The race starts at 7:30 and ends at 9 a.m., when the bridge reopens to traffic that has been backing up at each end.

That means that unlike, say, the Boston Marathon, where runners theoretically can continue to make their way over the course at any speed they can manage for as long as they like, the Seven Mile Bridge course closes right on time. Everyone has to be off by 9 a.m.

The administrators of the race have devised a uniquely unforgiving method of ensuring that this deadline is enforced. At 8:10 a.m., three school buses — each staffed by a driver, a race volunteer and a local law enforcement officer and known as a “shag bus” — set out very slowly across the bridge. If a bus catches up to a runner, the runner is required to board the bus.

Shag buses, or their regional equivalents, are an unhappy reality of many races where the needs of runners inevitably clash with the demands of automotive transportation. Disappointment comes in many forms throughout the road-racing season.

Take the Big Sur Marathon this coming Sunday, which follows California’s two-lane Pacific Coast Highway from Big Sur to Carmel. It’s one of the most beautiful marathons in the world, but it is also absurdly hilly, and runners have a strict time limit. If they cannot reach the 21.2-mile mark in 5 hours 5 minutes — they’re out of the race and off the road. The authorities need the highway back.

The New York City Half Marathon, which took place in March, had its own road-related requirements. The last wave of runners had a little less than two hours to run 6 or so miles before reaching the FDR Drive, and if they went any slower, they were forced to leave. In the end, about 10 people were directed off the course at the Houston Street and 23rd Street exits so that the city could get the FDR Drive — a desperately busy road even in the best of times — back, said a spokesman for New York Road Runners, the race organizer.

Similarly, administrators in some trail races that follow multiple loops through the woods bar the slower runners — the ones unable to complete the first loop in a set amount of time — from even attempting a second loop.

It’s not always an exact science. In the case of the Seven Mile Bridge Run, any runners overtaken by a shag bus are considered by definition to be too slow to finish the race. They have no choice but to board. It is a ruling with no chance of appeal.

This year, the first group of unlucky runners was plucked off the bridge at about the 4.5-mile mark, with still more than a half-hour of racing time to go.

“There’s a lot of disappointment” and a lot of talk of “beating the bus,” said Janet Oechsle, a longtime volunteer. She recalled an earlier race in which fear of being snaffled by the bus inspired a group of women to purchase shirts printed with a tricked-out version of the slogan: “Beat the Damn Bus.’”

Raceday was hot and humid. From inside the bus, the slower runners seemed like meek prey being hunted by oversized predators ready to pounce on their weaknesses. They fanned out, loping off on their own as if they were scattering across the plains (or wherever it is that prey scatters), but resistance was futile.

“They’ll see us coming and they get a little extra wind, they go a little faster,” said Doris Hawkins, a longtime shag bus driver. “It’s kind of cute.”

Of course, she said, there are those runners — if you can call what they are doing running, when in fact they are clutching the side of the bridge, hyperventilating — who actually welcome the arrival of the shag bus.

“Sometimes you get there, and they’re just sitting on the ground, waiting,” Hawkins said.

In the second bus, a familiar dynamic began. The bus would slow down, and Westerband, the deputy, would hop out. He’d be as nice as he could, applauding as the laggardly runners boarded the bus and making jokes about how, had he been running, he would never have been able to make it that far, himself.

Sweating and blinking, the disgraced runners would drag themselves numbly into a seat, to be greeted by a round of we’re-in-this-together applause from their fellow shag-ees and by a participation medal from Wendy Midnight, the very psyched volunteer assigned to the bus. “Good job!” she cried, each time a new runner got on.

Most of the runners did not think they had done a good job at all, although they had a variety of excuses as to why not. One by one, they entered the bus, and one by one, they explained what had brought them to this low point.

“I missed eight weeks of training,” explained Paul Petek, who is 79 and has run the race every year since 2001, when he finished in 1:03:56, not that he is keeping track. “I went on a cruise, and then I got sick, and then I pulled a groin muscle.”

“I never make it more than 5 miles,” said Pat Bridges, whose progress across the bridge was somewhat impeded by her decision to run while carrying a tote bag containing water, sunblock and a pen (“in case anyone needs one”).

“I’m more of a golfer,” was the excuse offered by Jim Hudson, 64, who owns a real estate brokerage, “although I’ve been trying to run about 5 miles once a week, roughly.”

As for Melissa Pietruszka, a 40-year-old administrator in a pediatrician’s office who said she was not much of a runner, there were myriad issues.

Pietruszka said she and her friends had been trying to build stamina by “walking the beach” — in point of fact, the sidewalk adjacent to the beach — four times a week, 7 miles at a time.

Her preparation hit a snag when she realized she had misunderstood a key detail of the race. She thought she had three hours to complete it. (A photographer snapped her photo. “I hope you’re photoshopping 40 pounds off this picture,” she said.)

As the bus rolled inexorably toward the finish line picking off stragglers, Pietruszka and other passengers began to second-guess the necessity of being there at all.

Hindsight had led her to conclude, she said, that at the time and place she was picked up — 5 1/2 miles in, 25 minutes to go, or at least that’s what she thought — she could have made it to the finish line in time simply by walking briskly.

“They pulled us in too quickly,” Pietruszka said. While she had been tempted to argue, she went on, she had been cowed into compliance by what she said (incorrectly) was a reference to “armed guards” on the race website.

The bus stopped for the last few unfortunates, and Westerband cajoled them through the door, even as they tried to remonstrate with him. The end was so close, within reach.

Taking advantage of the distraction, a runner who had been quietly sulking at the front of the bus suddenly got up and sprinted for the door, slipping past Westerband. He proceeded to jog unimpeded all the way to the finish line, where he was greeted with applause from runners who had managed to complete the course without riding the bus.

There was a moment of stunned silence on the shag bus, as the passengers contemplated the unethical nature of the man’s action and wondered why they had not thought of it first.

“That’s what we should have all done! We should have gotten up and bum-rushed the officer!” Pietruszka said.

She did not really mean that. What she meant was: As God was her witness, she would never again find herself in this situation.

“I’ll do the race again next year,” she vowed, “and I’ll be more prepared, and I will not be on the bus again.”