Claire Squires from North Kilworth collapsed on Sunday on the final stretch of the 26-mile course.
She was running in aid of the Samaritans; her mother has been a volunteer with the charity for 24 years.
Her close friend Nicola Short climbed Mount Kilimanjaro with her last year to raise money for the RAF Association.
She has spent the last 24 hours with Miss Squires' family.
She told ITV Central they are devastated but are taking comfort from the donations pouring in for the Samaritans on their daughter's JustGiving website page. The total raised has now passed £400,000.
Miss Short described her friend as "inspirational and generous".
Fighting back tears, she said Miss Squires was always there to talk to when she was feeling down, and wished she was around to talk to now.
She said Claire Squires' family were so proud of her and wanted it known that, at the time she collapsed, she was on course for a personal best in the Marathon of three hours 51 minutes.