"I don't remember falling asleep but six hours later [wife] Tracey woke me up for the prizegiving ceremony," he says of the event that started at 5pm last Friday before finishing at 11.15am on Saturday.
To put his time in perspective, William Hunter, of Christchurch, was second in 20hr 25min 41sec while Irishman David Ryan, from Galway, finished third in 20hr 48min 08sec.
Sue O'Brien, an Australian, was the first female to cross the line in 20hr 52min 06sec, ahead of Kiwi Kim McInally (22hr 22min 08sec).
About 2200 other competitors formed relay teams to do 15km stints over 160km.
Thompson-Gray's support crew included wife and fellow architect Tracey, their 6-year-old twins Mia and Louie, parents Ray and Carol Thompson, and Napier friend Murray Tonks, who was simply content to "watch me suffer".
"I got a text from my dad who was really proud of me," he says of his retired schoolteacher parents from Kaitaia, who live in the Coromandel.
His father saw him to the finish line on Saturday.
Hopelessly addicted?
No arguments from Thompson-Gray who reveals he isn't sporty, taking up fencing in Christchurch but dabbling in volleyball, soccer and squash at school.
"I enjoy sport but I'm not very competent," says the former Taradale High School pupil.
Thompson-Gray, who is a Napier co-director of the architectural firm with Ian Nelson and a Wellington director Mike Davies, competed in his maiden marathon in Wellington in June 2011.
One of five siblings, he couldn't accommodate the thought of elder sister Gabrielle Thompson, of Christchurch, "going on about how she did a marathon [several years ago] at Christmas time".
Unfortunately the Wellington marathon (42.2km) left him some what disillusioned.
He was feeling fine up to the three-quarter mark but hobbled the rest of the way, bankrupt of energy.
That prompted Thompson-Gray to surf the website where ultra marathons beckoned, bearing in mind he had to fit his demanding schedule around his young family and work commitments.
"The great thing about ultra marathons is that not even the elite run it all the way.
"You can walk uphill and jog downhill to finish at your pace."
Starting well before dawn breaks, he's able to return home from 3km runs to see his twins greet him and the day.
His ground work began with a run from Napier to Waipukurau, starting at 3am and finishing in 8.5 hours.
Then it was Napier to Tarawera Cafe, Te Pohue, almost halfway along State Highway 2 to Taupo in 10.5 hours over 77km.
He competed in the Kaweka Challenge Marathon (50km) in February, a Bay run/walk event Architecture HDT co-sponsors, before taking on the Tarawera Ultra 100km event in Rotorua in March.
"I run about 10km/h on the road but on the Kawekas it's down to about 4 and a half," he says, alluding to the hilly terrain.
On Mt Taranaki, Thompson-Gray was taking the challenge in his stride in the first 12 hours, about 120km of the route.
"The pain got too much after that. It was searingly agonising on the quads ... so it hurts to walk and you can't run again."
He prefers to classify that stage of the race "my unhappy place" despite establishing a 7.5km lead on his rivals.
Admittedly he became a bit obsessed with the Irishman nipping at his heels, sending his support crew on duty to check where Ryan was.
The Irishman made some ground but fatigue set in as he slipped back about 12km with Hunter swallowing him up, too.
"All I wanted to do was finish the 160km race but with 80km to go I thought I'm in with a chance but it was still a very long way to go," he says with a laugh.
He found solace in the tranquillity of a starry night, retreating into a self-imposed cocoon of darkness, just a myopic headlamp robbing him of 20-20 vision of any twists and turns that lay ahead.
Unfortunately with sunrise came a modicum of depression, lifting a curtain on constant reminders of the undulations to a seemingly never-ending finish line.
"I had read about people hallucinating but I stayed pretty [focused]."
Remarkably Thompson-Gray didn't pick up any blisters because he had applied petroleum jelly before taping his feet and had also worn glove-like toe socks to prevent abrasiveness with the shoes.
Wife Tracey initially worried about him training in the wee hours of the morning but has become accustomed to it and enjoyed her shift as his crew member.
It's the Kawekas for Thompson-Gray next on February 1, offering myriad courses for runners and walkers.
However, he has qualified for the Western States 100 (miler, 160km) in California in June next year, by virtue of finishing under 15 hours in Rotorua.
Fingers crossed because he won't know if he's in until organisers pick competitors in a lottery, to be disclosed next month.
"The kids are excited about going to the States because they'll get to go to Legoland so they don't care what I'll be doing."
Is sister Gabrielle going to up the stakes this Christmas?
"She'll be fine. I won't say anything," he says with a grin.
