Runners taking part in a 10km event were mistakenly sent on a 3km detour after a marshal left their post.
Organisers believe 300 of the 1,200 competitors went an extra two miles during Sunday's Bournemouth Bay Run.
Bournemouth council, which runs the event, said it was investigating what happened to the marshal who should have directed runners at a turning point.
Runners were said to have "turned the air blue" while one described the event's organisation as "shambolic".
Hayley James, who is four months pregnant and from Poole, said: "To have a race of that scale with only one marshal on a point is inexcusable.
"We saw loads of people walking at the end, I felt so sorry for them - I felt like crying at the 10km mark when my husband told me I had to run a further 4k."
Andy Isaac, from Bournemouth, said the event was "mayhem" with one point where an elderly woman managed to drive on to the route and was flashing her lights at oncoming runners.
Runner Kirsty Weston, from Wimborne, added: "I was really quite shocked [by the route error], it's inexcusable... you just expect more. The whole thing was shambolic."
Spectator Steve Shuck said: "Whether the marshal got caught short, went home or got fed up, they weren't there so the runners went on past that point.
"Runners were angry, I knew a lot of people taking part; the air was pretty blue."
But Trevor Finnis said it was important to put the mistake into context.
He said: "I'm sure this won't happen again and I'm just remembering that at the end of the day it was for a good cause."
Competitors who think they were affected have been told to contact the council for a "goodwill gesture".
The overall event, which also included a 1km, 5km and half marathon race, attracted more than 3,000 runners.
Jon Weaver, head of resort marketing and events at the council, said: "Unfortunately there was some confusion with marshalling arrangements at one point, but it was a critical point. We apologise unreservedly to those front runners.
"In 33 years of running... this is the first time this has happened and as part of our debrief we will be analysing the arrangements carefully... to learn for 2016.
"We understand runners have trained for a long time for the event and it's hard for them and we do empathise with how they are feeling."
It is hoped this year's event will raise more than £70,000 for the British Heart Foundation.