Sinclair was inspired by her friends who completed the series last year and decided she wanted to take on the challenge.
"I am feeling a little apprehensive about the terrain but I'm looking forward to being part of a Cadence team," says the 44-year-old mum.
"It should be a great adventure and fun."
Thompson and Sinclair, in many ways, are the face of the new type of athletes taking on trail running.
They will be among the 1000 athletes of all abilities beginning the popular Xterra series tomorrow morning at Riverhead Forest.
In the past, these events have attracted more serious athletes but now entice an increasing number of beginner runners and walkers.
The series is in its 11th year and has grown significantly since it started. Participant numbers have increased from about 50 people per event in 2004 to an average of 1000 per race this year.
"It's been a privilege to witness everyone that has found these events and taken part in them over the 11 years," says Dave Franks, the event manager from Total Sport.
"So many of them have been experiencing their first-ever trail run - they just look so alive and just loving life when they cross the line, it is the reason we do what we do."
Franks said the Xterra organisers realised early on that people were time-poor and they needed to meet that demand.
"We put together a schedule where you can have an awesome morning out, spot prizes are awarded as people cross the line, there is no hanging around for prize-giving. Grab a complimentary beer and hot sausage and still be home by lunchtime if you want to be."
The Xterra Team's Challenge is once again proving popular this year. A trophy is awarded to the team with the most points at the end of the series. It is as social or competitive as athletes want to make it.
Simon Fisher, from Partners Life, is entering a team in the challenge for the third time.
"The teams challenge is fantastic to create some friendly rivalry between staff in a fantastic environment outside of work," he says.
"The way Total Sport structure it is great as it caters for all team members, as the teams have people running or walking from the short course distance right up to the super-long distance."
Thompson echoes that sentiment. She says the Xterra series suits her well because it is more low-key and is less intimidating compared with bigger running events like the Auckland or Rotorua Marathon.
"You start to recognise people once you do more and therefore it's easier to strike up a conversation," she says.
"Each event has its own uniqueness in terms of scenery, challenges, atmosphere."
Thompson entered four short-distance events (5-8km) last year and this year she is taking on the mid distance (10-14km) in all of the seven locations around Auckland.
She said entering the Xterra series has given her added motivation to get into her training.
Meanwhile Sinclair, who has three young children, says it is always difficult to fit in time for fitness. She is fortunate to have the support of her husband, Glen. "I've learned to exercise when I have an opportunity even if it does mean putting off the more appealing glass of wine," she says.
Thompson advises older athletes who are looking to stay fit and healthy to start off slow with short distances but to exercise regularly.
"Find an activity that you enjoy and look for groups to join. Don't worry about what people might think about you - it's more important what you think about yourself.
"Celebrate your progress and encourage others on their fitness journeys as well. Embrace the outdoors, whatever the weather. Life's too short to spend it on the couch."
Sinclair believes busy mums should try to make fitness a priority.
"Fit in what you can when you can, and stick at it. Accept that you are going to have to be flexible and some weeks it seems near impossible, but don't give up. It's so worth it, you'll feel better," says Sinclair.
"I've really been enjoying my running and the camaraderie of the Cadence Group Training."
