The event - his second full Ironman after stepping up from his usual half Ironman - consisted of a 3.86 km swim, 180.25 km of cycling followed by a marathon (42.2km) run.
Desroches told Great Vegan Athletes: "It was my second attempt at the Ironman distance so one of the big goals was simply to complete the distance but I knew that I was capable of doing 8 hours 45 mins.
"I'm very pleased with the result. I was surprised to finish in 8:28 which is faster than I dreamed of going! But obviously, I'm competitive, and being passed by someone when there was less than 10 km to go and losing the podium was slightly disappointing - but I'm still really happy with the result."
His time of 8:28:40 placed him fourth of 2,271 athletes overall, and also fourth of the 14 male competitors in the Pro Class. He was the fastest Male Pro in the swim. He was also just four minutes 49 behind second place.
Good mental state
"I was in a really good mental state going into the event. I was excited but not too nervous and I knew I could execute a good race," he says.
"Fuelling is a big part of racing an Ironman and I was really glad I was able to fuel properly without having any stomach issues, which are really common in Ironman races.
"For the last 10 km my quadriceps were really tight and it was really painful just to move forward so I had to dig really deep but it was all worth it!"
According to Desroches, he was able to put in what he describes as a 'massive amount of training' - which went smoothly aside from a hip injury.
He said: "My plant-based diet definitely help since I was able to recover from seven hours training day (even a nine-hour day) really well.
"Recovery is key when you are training a few times each day so how I fuel has a big impact on my performance."
That diet has been simple and nutritious. "Lots of fruits (especially bananas and dates), nuts, vegetables, food fats (avocado and nuts) and I don't shy away from sources of carbohydrates such as bread, rice, quinoa and potato since I'm burning lots of calories and need to properly fuel my training," he revealed.
Desroches studied nutrition at University and has been vegan since 2013.
In 2016 he explained: "I stopped eating meat for animal ethical reasons but my reasons for going vegan were mostly environmental. I watched the Cowspiracy movie and I immediately decided to become vegan."