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23 March 2019, 10:14 | Updated: 23 March 2019, 10:27
Des Lally, 43, began his challenge of walking the mountain in the Brecon Beacons on March 28 last year.
He has juggled the feat around his normal life as a full-time mortgage broker, and with the support of his family and three children.
Mr Lally has so far raised more than £40,000 for two charities - Help for Heroes and Cancer Research UK.
"I was inspired to take up this challenge by my mum and dad who've have had three different types of cancer between them, and my dad spent 26 years in the Irish Guards, so that's the family connection," he said.
"It's just something I wanted to do to give a little back.
"I wanted to do something a bit different and unusual that would raise the profile of the charities - but I wasn't expecting so many other people to take my personal challenge to their hearts.
"The amount of support I have had both in terms of donations and also people coming to cheer me along and walk with me has been incredible - they have spurred me on to finish the challenge and I am really proud to have raised so much money for two amazing causes."
Mr Lally has often walked late at night or early in the morning in the dark, battling through the pain barrier and enduring weather conditions from gale force winds and snow to high temperatures.
In the summer, he had to suspend his challenge for 10 days after suffering heat exhaustion.
To complete the challenge on time, he has completed the walk multiple times in 24 hours - doing it four times on one day.
He has worn through seven pairs of trainers during the challenge, in which he has covered around 1,360 miles.
Many people have joined him including schools, choirs and community groups, as well as celebrities including Tour de France winner Geraint Thomas.
Shelley Elgin, community recovery manager at Help for Heroes Wales, said: "Des epitomises the spirit of our amazing supporters, going above and beyond to give something back to the Armed Forces personnel who have put their lives on the line for us.
"The work we deliver to support those whose lives are affected by their military service wouldn't be possible without people like Des giving their time so selflessly. We are very grateful."
On Saturday, Mr Lally will complete his final ascent with his father, 78-year-old Des Lally Snr.
He served for 26 years with the Irish Guards in Germany, Hong Kong, Singapore, Malaysia, Belize and Canada before retiring in Brecon in the 1980s.
Mr Lally Snr, who has had treatment for cancer and a heart condition, will climb Pen y Fan for the first time since his days in service.
They will be welcomed at the peak by a cohort from the Royal Welsh Fusiliers military band.