The ex-service personnel on the trek, organised by Walking with the Wounded, were due to cross the Omani desert until their plan was set back by the coronavirus pandemic.
Instead, the team of six former soldiers walked 300km in stages across the UK, starting at Pen-Y-Fan in Wales and finishing at the Anglo Omani Society in central London on Thursday.
Harry who is patron of Walking with the Wounded, rang the team as they neared the finish line.
He told them to “lean on” the experience of the walk when they feel “a little bit dark” in future.
“Guys just remember, you have got to hold on to this moment, you have got to hold on to this feeling,” he said.
“Because it doesn’t matter where you walk, or what you are doing. When inevitably we all end up feeling down, a little bit dark, in the weeks, months and years to come, you will remember back to this. Lean on this experience to pick yourselves up.”
Meanwhile, Harry has also been judging the WellChild photo competition, Hopes And Dreams: My Life Through A Lens.
Children were encouraged to enter photos illustrating what life is like for families living with serious health needs.
Ruby Smallman, 13, from Liverpool, won the competition with her photo Hope In An Oak.
Harry said: “The children and families I’ve had the honour of meeting over my years working with WellChild have shown incredible optimism, courage, and resilience.
“I wasn’t only proud to participate as a judge in this exhibition, I was deeply moved by each and every photograph as they capture a moment and say so much about their personal story.
“Every person who is part of the WellChild family is a true inspiration.
“Congratulations to all the entries and a special cheer to the winners.”
The competition celebrates the opening of the WellChild Art Auction 2021, supported by Christie’s, which is live online through global platform Artsy at https://www.artsy.net/auction/wellchild-benefit-auction-2021.
The auction will sell works from contemporary artists to raise money for seriously ill children.