Alice Dearing described herself as broken after becoming the first black female swimmer to represent Britain at the Olympics.
Facing 30-degree heat in the Tokyo water, she finished the 10-kilometre course in 2:05:03.2, some five-and-a-half minutes behind gold medallist Marcela Cunha in 19th place.
After the race, which began at an electric pace in the searing heat, she said: “I’m pretty broken, it was really hard – a really tough race. I had a lot more to give than that, I’m better than that, so I’ll check over what went wrong and look forward to the next one.
“It’s bittersweet – it’s incredible to race at the Olympics but I’m really disappointed with the result. I know I can do better.”
The 24-year-old has already set her sights on the next Games in Paris in three years’ time.
She said: “I want to go to Paris and have a better race, perform better than this. I’ve got a lot more to give and learn, this is my first Olympics. I’m definitely not done. I’ll go home, take my time to get over it and then come back stronger next year and the year after.”