The Cardiff Marathon has been put back 12 months with organisers now planning to stage the race in April 2018.
Run 4 Wales had announced Cardiff would host a full marathon in April 2017.
But 10 days after making that announcement, organisers say they will "not rush things" as they plan a route.
"Rather than try to stage the event in 2017, we feel it is more important to provide the perfect course for runners of all abilities," Run4Wales' Matt Newman said.
"There are a number of opportunities that need to be further explored in order to deliver an iconic route for the runners to enjoy.
"Having fully explored all the possibilities with Cardiff Council and Vale of Glamorgan Council regarding the best possible route we have decided not to rush things."
Chief executive Newman said there was massive demand for the full 26-mile event in Wales after 22,000 runners took part in this year's Cardiff Half Marathon on 2 October.
"It is obvious there is a huge desire from the running community within Wales and beyond for a quality marathon event," Newman added.
Cardiff Bay will host an event in April which Newman said "will give everyone a flavour of what we want to deliver a year later."
"This will include training events at venues around the course, information for runners and a general build up to what we hope can become a huge day in the Welsh sporting calendar," he added.