Welsh singer Katherine Jenkins said she was running the London Marathon in memory of her father and to show solidarity to people affected by the bombings in Boston.
She said: "Like everyone else, it was just so devastating to see that on the news. But I think we all feel more than ever that we want to do this and show our support for Boston."
The 32-year-old, who has raised over £22,000 for a cancer care charity, added: "I wanted to run it in support of Macmillan. My father passed away from cancer when I was 15 and the Macmillan nurses came and they were wonderful."
She said: "I really don't think of myself as an athletic person at all. I never in a million years thought I would do this. I'm glad that I am, but I am nervous."