Katie Burgess's son, Ben, needed open heart surgery just days after being born. Doctors discovered an issue with his heart during the 20-week scan and warned Katie the condition was 'not compatible with life'.
Katie said: "Doctors said you've got half an hour with your baby and then I'm taking him down to intensive care.
"You had to hold your hands behind your back because you just wanted to grab hold of the cot and stop them from taking him. But we had met the surgeon and the anaesthetist and in some ways, it was easy because it was over to you guys. It was out of my control."
Ben's heart surgery went to plan and five days later he was discharged from Bristol Children's Hospital.
Katie says she cannot thank the hospital staff enough for saving Ben's life.
She said: "They're just our heroes, they were just absolutely incredible. Everybody was amazing but especially the nurses - they had such a human element. When I was having a bit of a breakdown they just sat and held my hand."
Katie and her husband Stu are running the half marathon for the Grand Appeal and have raised more than £3000.
They hope the money can be spent on more comfy chairs for breastfeeding at Bristol Children's Hospital and some better coffee for staff.
15,000 runners are expected to take to the streets of Bristol on Sunday for the half marathon and 10km races.