Alex and Amy Fawcett, both 35, of Stannington, aimed to raise Â£11,000 for the North of England Children’s Cancer Research Fund (NECCR) through a series of activities throughout 2016.
Having completed a wide range of walks and runs across the UK and even in Abu Dhabi, they have entered Sunday's marathon having already tripled their total.
Amy said: "We’ve smashed it, we are on Â£33,000 now. It’s been a really busy year. We’ve done lots of challenges and this is our last one. It’s been amazing, people have also done their own challenges and events."
Their son Joseph, four, was diagnosed with a very aggressive brain tumour called atypical teratoid rhabdoid tumour (AT/RT) when he was eight months old. Brain surgery was unable to remove the entire tumour and Joseph then went through 11 rounds of chemotherapy, high dose chemotherapy and a stem cell transplant.
Amy said: "He has scans every six months, the latest one was a couple of weeks ago and it’s looking good. We never could have dreamt that he would be doing so well."
The Kielder weekend is run by Events of the North, a company led by Olympians Steve Cram and Allison Curbishley, in conjunction with Northumbrian Water, Active Northumberland, and Northumberland County Council.
The curtain raiser will be the Kielder Walking Event tomorrow, in which 175 walkers will complete two, four, and six mile routes in and around Kielder, following a successful pilot in 2015.
On Saturday, it’s the turn of the 10k and Run Bike Run competitors, followed on Sunday by the Half Marathon and Junior Runs.
The main event on Sunday gives runners the opportunity to tackle an almost entirely off-road course, which loops around the shores of Kielder Water; Europe’s largest manmade lake.
Race Director Steve Cram said: "The Active Northumberland Kielder Marathon always attracts a wide range of entrants, running for a huge array of reasons and causes. Amy and Alex are a great example of people who put their bodies to the test to support a fantastic charity and to raise Â£33,000 before they have even set foot in Kielder for this final race is an incredible achievement."
David Hall, head of leisure strategy at Northumbrian Water, said: "People come from far and wide to take part in the Active Northumberland Kielder Marathon, but often the most inspirational are from right here in Northumberland. We are all right behind Joseph and wishing him well and will be cheering on for Amy and Alex during the marathon."County Councillor Val Tyler, cabinet member for arts, leisure and culture, said: "What a stunning achievement this is by Amy and Alex. They started out with a very ambitious fundraising target, and they have more than tripled it through sheer determination and by raising awareness of a very important charity. We wish them a very successful marathon weekend, which concludes their year of fundraising, and we wish their little boy a full and speedy recovery."
Anybody wishing to support Amy and Alex can do so online at www.justgiving.com/fundraising/Josephsappeal