Joey Hawke, 31, said knowing his girlfriend Emily Harrison, 28, was waiting for him at the finish line helped him pull through the final stretch of the challenge while battling an infected leg.
And stunning pictures captured the emotional moment Joey surprised Emily by getting on one knee and asking her if she would marry him.
Emily said she had "absolutely no idea" he was going to propose.
She was stunned when friends suddenly held out a banner which read ‘Emily Rose will you marry me?’.
When she turned around Joey was down on one knee to pop the question.
Joey had just reached Robin Hood’s Bay near Whitby, on May 28 following a five-day walk from St Bees on the Cumbrian coast before the grand romantic gesture.
The newly engaged couple then threw pebbles taken from the Irish Sea where the challenge began and threw it into the North Sea – a tradition for those who complete the coast-to-coast trek.
Joey, from Padiham, near Burnley, said: “It was an unbelievable feeling, one I’ll never ever forget. I was battling a leg infection and had blisters during the walk, but I knew that when I finished, I would be getting on one knee to propose to Emily.
“I knew I had to do it and knowing she was there waiting for me pulled me through the last day. It was very emotional walking down, the drop into Robin Hood's Bay is very steep and you can see everything as you approach. It was an incredibly special day.”
Joey, who works at Padiham Fire Station, said he wanted to take on the challenge to raise funds for The Firefighters Charity and the Royal Navy and Royal Marines Charity.
Along with four friends, he trekked for up to 16 hours a day and crossed five national parks while wearing 45lbs of protective gear.
They completed the walk in four days, 13 hours and 55 minutes – but Joey was left in crutches for two weeks after the infection spread through his leg.
But despite the pain, he said May 28 will always hold a special place in his heart as he married the love of his life and raised nearly £4,000 for charity.
He said: “The challenge was the hardest thing I’ve had to do in my life, just getting yourself motivated to do another 38 miles but it was all for a good cause.
“I only found out afterwards that I had quite a bad infection, and if I didn’t get it treated, I could have had sepsis. I had to be put on antibiotics and spent two weeks on crutches afterwards.”
His fiancée Emily said that she was "crying" throughout the emotional day and couldn’t even drink any prosecco as her hands were shaking so much.
She said the moment was "so unexpected and bizarre" but added that Joey "hit it out the park".
The dog walker said: “I was so invested in what he had just done so I was already so emotional watching him walk to Robin Hood's Bay. I thought to myself he was being a bit funny and now I realise it was nerves but he was being odd.
“But I never would have thought he was going to propose. It was all a bit of a whirlwind. It was so overwhelming. I definitely ugly cried a lot. I remember someone handing me a glass of prosecco and I was shaking so much the prosecco was all on the floor I didn’t have a sip.”
The couple met in 2016 through a family friend and planned to meet in Manchester for drinks, but Emily had to stay back longer for work and Joey thought she was "fobbing him off".
When she was finally ready to go, they realised they lived nearby and ended up meeting in person for the first time at a bus stop.
Emily added: “I’ve never met anyone quite like him, he dedicates himself to these challenges and he always perseveres. I’m so proud of him. The relationship we have is just we’re best friends.”
Click here to donate to Joey’s fundraiser.