He joined up with Tauranga-based coach and former Commonwealth Games marathon runner Craig Kirkwood, and has since improved his running.
Clark led for 20km in the Tarawera Ultramarathon in March and finished the Rotorua Marathon earlier this month in 2hr 39min, good enough for 14th place.
He has plenty of confidence knowing he is on the right track.
"Craig got me on track for the marathon and has organised a few really painful sessions," Clark said.
"It's been a good start to my season, getting some quality training under my belt under the guidance of a quality coach. He's an old-school runner who knows it's only going to pay dividends if it's going to hurt.
"I'm really targeting a big performance at next year's Coast to Coast and Braden Currie set the bar really, really high on the run leg this year and showed what needs to be done.
"Competing in the marathon is part of that, trying to raise my basic running speed to get close to the likes of him."
Though he hasn't raced a bike since November's Tour of Southland, Clark has also noticed the increased running focus has boosted his performance on two wheels.
He has also completed the King of the Harbour surf ski race and the New Zealand marathon kayaking championships at Lake Karapiro recently.
The 3D race next Sunday features a 9km kayak leg on Lake Tikitapu (Blue Lake) followed by a 30km mountain bike leg through the Whakarewarewa Forest and an 11km forest run to finish.
"It's probably the most fun multisport course in New Zealand at the moment and the mountain bike leg would arguably be one of the best in the world.
"The nasty, horrible, hilly run at the end to finish things gives it a little bit of everything and this race will be a good indicator of where I want to be for the next six months."
Although defending champion Dougal Allan (Wanaka) will miss the weekend there's plenty of competition in the men's race. Clark will be up against Ussher, Trevor Voyce (Nelson) and Luke Osborne (Hastings).
Meanwhile, two-time winner Elina Ussher, Rotorua's Nic Leary and Christchurch-based Dane Sia Svendsen headline a strong women's field.
The dark horse in the female ranks could be New Zealand surf lifesaving representative Danielle McKenzie.