London Marathon runner Dawn Williams had to run the event in odd shoes after her trainer was lost during the starting mayhem.
Dawn, of Banbury, lost her shoe at the start of the race when another runner caught the back of her heel.
A kind spectator handed the 46-year-old her own trainers but Dawn could only take one as her other trainer was the one carrying the time chip she needed for the race.
She said: “She was just a spectator and she took the shoes off her feet.
“I don’t know who she is and I have no way of giving them back.
“I’ve raised £600 for the Banbury Sea Cadets and I was panicking, thinking ‘I can’t pull out’. It felt horrible running in different shoes but I’ve got no blisters.
“I was so grateful to her. She just went above and beyond. I would love to know who she is so I can give her the trainer back.”
Dawn was unable to get her lost trainer as thousands of runners were pouring forward at the start of the event. She spoke to a steward who said she would have to wait until all the sections of runners had set off 45 minutes later to look for the shoe.
Another spectator also offered Dawn a pair of shoes and ran to her nearby home to collect them, but as these turned out to be fashion pumps she did not think it would be a good idea to wear them.
Despite a 15-minute delay, Dawn managed to complete the race in three hours 45 minutes – just five minutes slower than her original target.