John Keep, of St Stephens Gate Medical Practice, in Norwich, was taking part in the Splat Quack Go charity race in Woodbridge, Suffolk at the time.
The 37-year-old was just minutes from the finish line when he saw a competitor being treated by paramedics.
He said: “I could see the patient was being helped so I thought I had better check. When I got there, I could see it was possibly quite a bad fracture.
“Given the location it was going to be a while for an ambulance to come so I thought it best to stay.”
The patient, who suffered a lesser injury than first thought, was taken to hospital from the event on Saturday, June 16.
“I was glad to help but I did see my wife Lydia pass me and she now has bragging rights at home as the only official finisher,” Dr Keep added.
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