The 31-year-old, who is taking part in a 250-mile charity run for Women's Aid, revealed the assaults were carried out by her athletics coach when she was 15.
"I got quite close to the running coach because you do," she said.
"You start to trust them when you're a young kid, and he started to take me for a few private sessions because he said I was good and I could do with some extra sessions because that would really help.
"He abused the fact that I was an innocent person who wanted his support and his compassion and his care as my running coach."
He was a male role model to me," Webster added.
"You should never touch a young girl anyway, but he very, very manipulatively and very slowly sexually assaulted me."
"I wasn't very confident.. not one time in my head did I think I was being sexually assaulted'"
"Not one time in my head did I think 'I'm being sexually assaulted', because if I did, I would have done something about it."
Admitting that it was only due to another younger girl who reported the abuse that she too contacted the police, she added:
"I'm here [speaking on radio] as well. It might help someone else bring it to light. I want to break the taboo about abuse as a whole."
The man was later sentenced to 10 years in jail and put on the sex offenders register for life.