Only the winner, Jake Harrison, completed the correct course in Sunderland.
But Harrison had built up a sufficient lead of the race that the vehicles following him were out of sight.
The second- and third-placed runners headed off in the wrong direction, and that meant that everyone thereafter did not complete the full distance.
For those runners who fell short of the 26.2 mile route, it meant that all their months of training and effort did not result in a complete marathon.
Organiser DS Media & Events has issued a statement of apology, and has offered affected runners a 25% discount on next year's entrance fee.
In a statement, it said: "Only the lead athlete followed the correct route through the Sheepfold area near the Stadium of Light.
"Unfortunately, the second and third placed runners were not within line of sight of the leader and lead bikes and vehicle.
"This resulted in all those who followed taking an incorrect route through this section."
But participants are not happy with the offer of compensation.
"Been thinking about the Marathon of the North debacle," one said on Twitter. "Think it’s an insult to offer 25 per cent off race entry as recompense.
"People train for months and have had a hard earned achievement taken away from them."
Astonishingly, though, it's not the first time in recent years that a distance-race has had troubles with length.
The Cardiff half-marathon of 2010, featuring 15,000 runners, made a last-minute change to the route and ended up with a course 193m of the correct distance.
Source: Newcastle United Mad