Inspired by ultrarunning book Born to Run he went barefoot from the start, soon moving on to Vibram Fivefingers shoes.
"I always preferred to run in less," he said. "It seems to improve running form and they are a lot lighter on the feet."
After ticking off that half-marathon Keenan wanted more of a challenge and quickly progressed to the extreme distances.
"I got hooked on running and I found that I am a bit of an all-or-nothing person."
Keenan finished third in his first ultramarathon, a 60km race last year. In November he won a 160km race in Taranaki, setting a new course record in the process. He also won a 60km in Wellington, with those results revealing a natural aptitude for ultrarunning.
"I found some early success in longer distances, that's where my strengths lie."
Keenan recently moved to Rotorua and has been taking advantage of the running trails around the city.
"I am absolutely loving it, particularly the Redwoods."
A senior sergeant with the Rotorua police, he said fitting 160 to 200km a week training around shift work and family could be difficult. He usually ran to and from work and admitted colleagues sometimes thought he was a bit mad - "especially when I am running home at five in the morning after working a night shift."
Keenan said he had originally aimed to beat nine and a half hours tomorrow but inflammation of the achilles has hampered his buildup.
"I am nursing that at the moment," he said. "It's all about how that is going to hold up."
Regardless of his injury, he is looking forward to competing alongside some of the top runners in the world.
Keenan will do his first Rotorua Marathon in April - surely a walk in the park after tomorrow - but ultramarathons are where his ambitions lie. "I would love to do some New Zealand representative stuff overseas. Hopefully I will be doing that later in the year."