The former Love Island star took part in the 7-hour run, 50km run in aid of Big Moose, a UK based charity that provides therapy for people struggling with their mental health.
He joined a group of runners on their second day of a five-day, 250km marathon.
Taking to Instagram, the Limerick man opened up about his experience.
“I completed my first Ultra-marathon,” he began.
“I’m genuinely in awe of @lewis_robling and this group of amazing people. I’ve been fortunate enough to get to the top of sport but never have I come across people with such positive and determined mindsets,” he said praising the groups leader.
“250km is a different beast and I was kindly welcomed to run with this group on their second day of 50km. I hobbled over the line after 7hours of running, with two twisted ankles and a lot of learnings about myself and my mental capabilities,” he said.
“I genuinely wanted to give up after 34km but this group pulled something out of me that I didn’t know I had and that’s the real beauty of it all. We never know what we’re capable of and sometimes all we need is somebody to support us!”
“@bigmoose.charity is doing exactly that by providing mental health care for people struggling and also helping to prevent suicide,” he explained.
“I’m very grateful to have met these lovely people and to play my part in raising money for mental health. If you can afford it please, any donations would be greatly appreciated and will genuinely help to save lives,” he added.
Earlier this year he opened up about his “really, really bad” mental health struggles after winning Love Island.
The reality star and rugby player was at one point contemplating suicide as everything became “overwhelming.”
Speaking to Doireann Garrihy on her podcast ‘Laughs of your Life’ he said: “I was alone in London and I didn’t have a career, didn’t know what I was doing.”
He said was under pressure to “sell” himself after appearing on Love Island in 2019 and stealing the show with fan favourite Amber Gill.
"I remember one time I was back in Dublin and I was driving around and it got really, really bad,” he said.
"It got overwhelming at one stage, I was behind the wheel of the car and I was like, ‘alright this is going one of two ways.’”
"I don’t want to get too dark on the podcast but it was basically a decision of, alright, either this is all over, or bring yourself to the doctor right now.”
The rugby Olympian sought professional help and went on “pretty heavy medication”.
He told Doireann that no one but his mother knew what he was going through.
“I sat down with a piece of paper and I started writing down the things I cared about, my family, my friends, fitness, presenting,” he said.
"I decided those were my pillars, I’m going to surround myself with my family and friends, I’m going to do my best presenting and hosting and I’m also going to create a fitness app and I’m just going to go hell for leather.”
Greg recently landed his dream role as a TV presenter, taking over from Martin King on the Six O’Clock Show on Virgin Media.
“I’ve been wanting to tell you this for so long and now I finally can… I’m going to be a presenter on the Six O’Clock Show,” he announced.
“This is an absolute dream come true. I’ve done a couple years of presenting but I never thought I’d actually have a show. I’m so excited to start this journey and it’s made even better I’ll be beside the lovely Karen Koster.”
“Also big shout out to Martin King for such incredible work. You are an inspiration to me and I’ll never be able to fill your shoes but hopefully I’ll make it my own and do you proud on the show,” he gushed.
“Of course thanks so much to you all for your support as always, I can’t wait to start this new chapter!”