Selly Rowles, 32, was running in the five-mile road race herself when, after about a mile, she noticed a disturbance up ahead and some runners shouting for a marshal.
She realised 67-year-old Northampton man Tom Sullivan had collapsed in a ditch.
She stopped and checked his pulse but could not find one.
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Selly, who has been a special constable with Cambridgeshire Police for four years, and three other competitors assessed him and began CPR for about 30 minutes before an ambulance and air ambulance arrived.
Selly said: “The training I had been given as a Special came back to me immediately and I thought, ‘I can do this’.
“I went into autopilot and the training took over.
“I think he was gone for a while and he was changing colour.
“He would start to draw air in and then he was gone again. He was with us and then he wasn’t.
“We knew he was breathing before we left the scene but I had no idea whether he would survive until the following day when I saw he had woken up in hospital.
“It made me feel very, very good. It was just sheer happiness knowing he was back with us.”
Running just behind Tom on July 31 was his 60-year-old wife Mary.
She said: “The Red Cross said what they did had to be done immediately because it kept him alive until they arrived and used their defibrillator.
“They saved his life. Everyone has told him he was a very lucky person because the only chance he had was to have someone right there who could do CPR.
“I would like to thank Selly and everyone else who helped.
“But thank you just seems inadequate, it’s far more than that, they literally saved him and gave him back to us.”
Tom has now had a defibrillator fitted and is out of hospital recovering.
He cannot drive for six months but should be able to start running again eventually.
Selly said: “I was told that what we did before the ambulance arrived had kept his blood circulating and kept him alive.
“That was when I realised that what we did had saved his life.
“I would have done it for anyone and I hope someone else would have done the same if it was me.”