Terry Rosoman, 38, from South Wales, trekked through the mist and pitch-black conditions across the Brecon Beacons in a 75-mile phallus-shaped route, to raise money and awareness about men's mental health issues for Movember.
Mr Rosoman, who began his journey at 5pm on Friday at Abergavenny railway station and finished at about 4.30pm on Saturday at the same place, described the challenge as the "hardest thing" he has taken on.
Speaking in his car as he drove home after completing his run, he told the PA news agency: "It was absolutely horrendous.
"It was really tough, really hard, the hardest thing I've ever done in my life. It was just a hard slog for 24 hours.
"I didn't sleep, I ran through the night, pitch black, I couldn't see anything. We got lost loads of times.
"The elevation killed us. It was really tough. I'm just glad to get it done."
Mr Rosoman, who is a director for a freelance marketing company, ran the shape of the large manhood to gain the attention of his "target demographic", and joked that most men find the shape "hilarious".
The challenge was in aid of Movember, a charity raising awareness and funds for men's mental health, suicide prevention, prostate and testicular cancers, and marks its 21st anniversary this year.
He said creating the largest drawing of male genitals was his biggest challenge, and he has raised more than £3,500 of his £5,000 target so far.
He said the most difficult part was running in "12 hours of pitch black" through the night with little to no visibility because of heavy mist.
"I couldn't see anything, not a sausage. We've got torches on, but all you can see is what it highlights in front of you," he said.
"Even with the torches, you can only see a couple of feet in front of you because of the mist."
Mr Rosoman said the highlight was crossing the finish line at Abergavenny railway station to see his daughters and wife, who "ran up and gave me a hug".
"A lot of my friends and family were there who I wasn't expecting. It was brilliant," he said.
He has not yet set his sights on another challenge of this scale and instead intends to overindulge at Christmas with his family.
"I'm getting fat for Christmas. That's it now, no challenges, no training. My body is in bits. It's going to take a fair few weeks to recover," he said.
"I'm just going to enjoy Christmas and spend time with the family."
He thanked those who joined him on his challenge and people who donated to his fundraiser.
"Just thanks for all of the support I got from people, everybody who came to run out with me, donated and shared. We wouldn't have raised over £3,500 without them," he said.
Mr Rosoman hopes his endeavours will inspire men to take on their own challenges.
"I just wanted to promote this message of challenges, essentially," he said.
"Ultimately, it's just about having aims, goals, but grand aims and grand goals that are bigger than yourself, that are more important than going to the pub, and it will give you that sort of purpose."
To support Mr Rosoman's fundraiser, you can visit: https://uk.movember.com/donate/details?memberId=15174570#amount