Dawn Nisbet, 41, from Oldham in Greater Manchester finished last in the 5k run - ten minutes behind the other runners.
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But after 17 years of being inactive, Ms Nisbet said the picture shows "my joy and my sense of achievement", which has "touched lots of people's hearts".
She wants others to "stop letting what other people think of you" stand in the way of "what you want to do".
Ms Nisbet said her first thought on seeing the image, taken by photographer Val Lovatt, was "how embarrassing" because "I'm a big girl, there are lots of jiggly bits in that picture".
But now she thinks it captures "the sense of achievement" and "what I set out to do".
"When I looked at it I saw a very sweaty, very tired person... but nobody else sees that, they see that smile and that joy", she said.
Ms Nisbet, who works for the NHS, said "self-limiting beliefs" had affected her self-confidence for years, making it "really hard" to become more active.
"I've probably been inactive for about 17 years if I'm honest. I've done the odd flurry into trying the gym and various things like that.
"I just think that Catch-22 that the less you do, the harder it becomes.
"And then you don't start doing anything, you've got the self-limiting beliefs that 'well I can't do it so I won't do it' and that just festers inside and becomes this big blocker for you ever trying anything different.
"The messages in response to that picture has spurred me on", she said.
Ms Nisbet, who has two children, said she wants "to be healthier for them - and for me".
She said her daughter had posted on social media that she was proud of her mum and "that's a testimonial from a 15-year-old that I'm hopefully inspiring my daughters to go out achieve their goals as well."
An inspiring photograph of a woman celebrating crossing the finish line of a Parkrun has gone viral.