Limited spaces are now available at the weekend of events, happening on Saturday 6 and Sunday October 7, 2018, at Northumbrian Water′s Kielder Waterside and Reservoir site.
Places in the Half Marathon and Run Bike Run are already sold out, with the Marathon, 10k and the children′s events not far behind.
Last year′s events were full to capacity and race organisers are predicting a similar picture this year, urging anyone thinking of registering to take part in any of the events to do so now, so as not to be disappointed.
Race director Steve Cram said: "We′ve got people travelling from all over the country to this year′s events and the weekend just keeps growing year upon year.
"With just three weeks to go, it′s clear we′re in for another busy year. If you want to get your place on the start line for the marathon or 10k then you need to get your registration in now before it′s too late."
The Active Northumberland Kielder Marathon Weekend 2018 will attract more than 3,500 runners, to Northumbrian Water′s picturesque Kielder Reservoir and waterside sites in Northumberland, where the events are based.
Northumberland County Councillor Cath Homer, cabinet member with responsibility for arts, culture, leisure and tourism said: "Kielder is such a unique location and many hundreds of people will be travelling from far and wide to come to Northumberland to take part in the events over the weekend.
"This is fantastic for the county, the surrounding rural businesses and for the many charities that will benefit through fundraising."
The weekend is delivered by Events of the North in conjunction with Northumbrian Water, Active Northumberland, Northumberland County Council and Northumbria Healthcare NHS Foundation Trust.
For more information on the events taking place at the Active Northumberland Kielder Marathon Weekend 2018, and for the quickest way to enter, visit www.kieldermarathon.com.