Double Decathlon legend Daley Thompson was among those present at today's races, which included the 10k and Run Bike Run.
He said: "It's been fantastic, the weather's been perfect for the runners. Everybody seems to be having a good time and I've enjoyed watching from the side-lines, after trying out part of the route on my bike before breakfast."
Josh Fiddaman, from Blyth Running Club, took first place in the 10k, crossing the finish line in 35mins and 35secs. The winning female was Suzanne Thew, from North Shields Poly, with a time of 40mins and 57secs in the 10k.
In the Run Bike Run, the first relay team over the line was South Shields 1 with a record finishing time for a team of 1hour, 56mins 46secs.
The first male individual was Trevor Dixon and he did it in 2hours, 12mins and 48secs. Claire O'Callaghan was the first female individual through the Run Bike Run in 2hours, 33mins and 55secs.
The Active Northumberland Kielder Marathon weekend, which is in its ninth year, is delivered by Events of the North in conjunction with Northumbrian Water, Active Northumberland, Northumberland County Council and Northumbria Healthcare NHS Foundation Trust. Tomorrow, runners will take part in the marathon and half-marathon, as well as junior races.
Race Director Steve Cram said: "We've really kicked the weekend off in style today! We've seen some amazing running performances and I can't wait to see what tomorrow holds for our competitors.
"I'd like to thank everybody today who either took part, came to support their friends, or all our amazing volunteers who help make this weekend possible. The team are all looking forward to day two tomorrow and the half marathon and the main marathon itself."
1st male: Josh Fiddaman, Blyth Running Club - 00:35:55; 2nd male: Nicholas Hamilton, Achilles - 00:36:32; 3rd male: Matthew Costello - 00:38:56.
1st female: Suzanne Threw, North Shields Poly - 00:40:57; 2nd female: Claire Davies - 00:41:25; 3rd female: Rachel Breheny, South Shields Harriers & AC - 00:42:31.
Kielder Run Bike Run:
1st team: South Shields 1 - 01:56:46; 2nd team: South Shields 2 - 02:02:29; 3rd team: Sedgefield Specials - 02:08:32.
1st male: Trevor Dixon - 02:12:48; 2nd male: Graham Bradshaw, Durham - 02:14:42; 3rd male: Mark Southern - 02:15:26.
1st female: Claire O'Callaghan, South Shields Tri - 02:34:48; 2nd female: Pip Graham - 02:34:07; 3rd female: Tracy Henderson - 02:35:08.