Coached by her husband Stephen, Daphne, won the 75-79 year age-group after facing a 1,900 metre sea swim, 56-mile bike ride and a 13.1-mile road run.
The result was even more remarkable as she was the oldest participant in the competition out of 2,219 entrants at 77, and Daphne is delighted how it went.
She said: “It is the first time I have raced that distance in ten years so I am feeling extremely pleased as is husband Steve.
“There were no finishers in my category or the women’s age-groups of 65-69 or 70-75 years, only men finished in those.”
Daphne, from the Littlehampton area, only started entering triathlons from the age of 50 but has now been competing for 27 years.
The event which took place on Sunday, June 18 in Elsinore, began with a gruelling 1,900 metre swim.
She said: “It was very crowded in the water, I got a little bit beaten up but it is the same for everybody.”
In the next phase, participants had to contend with a 90km cycling course through a mix of city, coast line and countryside.
“The bike ride was wonderful even though it is 56 miles long I loved every moment.
“It starts out looking through the trees to the sea across to the very nearby Swedish coast, then into lovely countryside.”
Daphne’s final challenge, the running course, was her toughest, consisting of 3.5 laps of the city over a 21.1km distance.
“The run is the hard part and I confess to not having run a single run longer than six miles in training but I had tremendous support from both spectators and competitors.”
Daphne finished the event in an impressive seven hours and 43 minutes, comfortably under the competition’s cut-off time and has said she has no intention of quitting
She has already qualified for the sprint and standard distance races at Rotterdam’s ITU World Championship in September.
“I would carry on as long as I am still having fun and can get in inside the cut-off time.
“I still enjoy the training and the travel and I certainly like coming over the finish line.”