But, the following year, while out training one night, he fell into a ditch which put him in Waikato hospital for five weeks. Afterwards, running took a backseat for 20 years, although he remained active through tramping, kayaking and mountain biking.
In the summer of 1988-89 he walked the Kepler Track. At Rainbow Reach he and his twin brother Roger tossed their packs on to a truck. Roger said to him; "You're always concerned about getting enough exercise. After three weeks walking you're reasonably fit. Why don't you try running the last 10km of the track with me?"
Donnell reluctantly agreed, but found that he actually managed it quite well and enjoyed the experience. At the end, Roger encouraged him to resume running as a regular pursuit.
Phillip was not highly motivated, but a few days later an envelope arrived in his mailbox in Gisborne. In it was a YMCA Marathon Training Programme for Beginners, with a note from his brother: "You are going to run your first marathon with me in Rotorua on our 38th birthday, April 29. Follow this programme and you'll be all right on the day."
He did follow the programme - to the letter, and he did finish his first marathon despite torrential rain, cold temperatures, and shoes that were too small - the Silver Jubilee event at Rotorua in 3h 33m 27s.
He's been running ever since, clocking up 99 marathons and 115 half-marathons to date. He said he only intended to run one marathon, "so it's rather a pleasant surprise that I've managed to add a couple of zeros".
He has run Rotorua 21 times, and was admitted to the "Survivors' Club" in 2009. He has also run Auckland 13 times and been acknowledged as a "Roadster". Only 13 of his marathons have been in the South Island and two overseas - Gold Coast and Jerusalem.
"I have been fortunate to have had very few injuries over the years, and have been blessed with an understanding wife and supportive family," Donnell said.
If successful on Saturday, Donnell will become the 45th member of the New Zealand 100 Marathon Club, part of a global network. At present there a fewer people in the world who have run 100 marathons than have climbed Mount Everest, he said.
When asked what he likes about running, Donnell recites an acrostic poem.
"Marathon is a way to Massage your mind, Adjust your attitude, Replenish your resources, Affirm your affections, Track your time, Hone your heart, Own up to others and Nurture your nature."
