Nina Twigger, 40, who works at NFU Mutual in Bridgnorth, said she was shocked when one of her colleagues lost her son through suicide last year.
Connor Stephenson, who was from Alveley, died on June 17 last year, and his mother says they still do not know why the popular 24-year-old took his own life.
Connor's mother, Rachel Stephenson said: "He had no issues that we were aware of. He seemed happy and had even made arrangements to meet some friends later that night. There were just no signs that anything was wrong."
She added that nine months on from the former Bridgnorth Endowed pupil's death, the family still have no clues as to why Connor decided to end his life.
"We still have no idea why. We have spoken to his friends and his work colleagues and just don't know.
"He was incredibly popular and so many people in Alveley were shocked by what happened."
She said while they did not know if Connor had ever called the Samaritans - the service is confidential - he had searched the charity on the internet.
"We don't now if he called but we do know he had the Samaritans in his Google search history. The whole message we really want to give out is for people to reach out for help if you need someone to talk to, and for those close to them, please double check on them if you are concerned," she said.
Having entered the ballot for the marathon last year, Nina discovered she had secured a place for the April 23 event in October, so decided to use it as an opportunity to support her work colleague.
Nina, who herself has a 13-year-old son, Ryan, said: "Where we work it is a small team and it is like a little, close family so when something like this happens it effects you all.
"We all wanted to support Rachel and when I entered the ballot, I didn't think I would have a chance of getting a place so was not thinking about raising money.
"When I found out I had a place in October, I approached Rachel and asked if she wanted me to raise money for Connor and she said raise it for the Samaritans."
Nina, who is a member of Bridgnorth Running Club and has run the London Marathon before in 2018, is hoping to raise £1,000 for the charity when she takes on the 26 mile challenge again.
She added: "If you are struggling, please reach out to someone. Don't keep quiet.
"If you suspect that all is not well with someone else, ask them how they are feeling and then ask them how they are really feeling again. It might just be the chance they need to open up."
For those wishing to support Nina in the London Marathon, her JustGiving page can be found at: justgiving.com/fundraising/nina-twigger2
If you need to speak to somebody at the Samaritans they can be contacted free, 24-hours a day by calling 116 123 or by texting SHOUT to 85258