Stuart Vince and colleague Phil Jackson were also trying to get hundreds of people to join the NHS Organ Donor Register as part of the Big Donor Challenge.
Last year, Stuart Vince donated his kidney to a friend who had needed dialysis for 16 years. After recovering, he decided to campaign to raise awareness of the donor register.
After they crossed the finish line, the two firefighters were met by Nicholas Crace who recently became the oldest living organ donor.
They had originally planned to run to Land's End and then kayak back but had to run most of the way on the return journey because of the recent bad weather.
Mr Vince said he found the toughest part of the challenge was the heat as they were crossing Dartmoor. For Mr Jackson, he suffered badly with swollen ankles and blisters. At one point he had to go to A&E to have his blisters taken care of.
Mr Vince and Mr Jackson started their challenge on 20th May and arrived back in Portsmouth this weekend. Mr Vince has been campaigning to raise awareness of the organ donor register since donating one of his kidneys to 20-year-old Michael Paxman.
– Stuart Vince, Big Donor Challenge participant
"Michael had kidney problems from a young age, and was on dialysis for 16 years. He was put on the national waiting list for a kidney in October 2010 but I wantedto help him personally so I offered to donate one of mine."
After undergoing tests and discovering Stuart and Michael were a match, Michael received his friend's kidney in June 2011 and has since gone from strength to strength.
Mr Vince then set himself the challenge of running 260 miles and rowing 220 miles and managed to persuade Mr Jackson to join him.
You can hear how they got on below: