"The marathon went on the back-burner."
Everyday life as they knew it changed as the family spent a year in Starship with Joshua undergoing surgery, six weeks of radiotherapy and four months of intensive chemotherapy.
"Joshua had a long road to recovery and so did I."
Now with Joshua four and a half years in remission, in January 2016 Mrs Gallagher made the decision it was time for her to "get well too" and rediscover her passion for running.
She said for so long her focus had been on getting Joshua well she had almost forgotten about herself.
Since starting training last she has lost 34kg and run more than 1700km.
"As mums we have to learn to make ourselves a priority. The big thing I found was that with me being happier, everyone else was happier."
Mrs Gallagher said when she started training she was "really unfit" and didn't want people to see her so worked out at Profiles Gym until she dropped below 80kg.
A real turning point was joining Lake City Athletics Club this year for their marathon clinics, which she said were amazing.
She said through the club she had been surrounded by some "fabulous examples" of people running into their 70s, learnt new tracks to run on and had made friendships.
Mrs Gallagher said getting into running again had changed her whole outlook.
"It has been completely life-changing, the person I am now in comparison to the person I was."
Come race day Mrs Gallagher said she hoped for a time under four hours, 45 minutes but "finishing is the big goal".
"I'm as prepared as I can be. It's all a celebration of what we have come through. Everyone is well, we've done it. I have a wee son who is a survivor."
Mrs Gallagher said her parents would also be watching, turning the tables on the times as a kid she would watch from the sidelines as her parents ran the marathon.
She wasn't sure how she would feel crossing the finish line.
"I'm so emotional about it. It's been a five-year goal and so much has happened in those five years."
