Steph Twell will compete in her first marathon in Valencia on 2 December.
Twell, 29, is switching from cross country and track, having won European 5,000m bronze and Commonwealth 1500m bronze.
She previously recorded seventh place at the Commonwealth Half Marathon Championships in Cardiff.
"At this stage of my career it just feels right to finally step up and challenge myself over the marathon distance," said Twell.
"I've always had ambitions to see how I fare over 26.2 and since becoming five times British 5000m champion on the track, I feel now would be a great time to dip my feet in the water to see if I can also have similar success on the roads.
"With two years to the Olympics, I'm aware it is no easy task to tackle the marathon (and qualify) but I have hopes to make my best impact at Tokyo 2020 and the marathon might be one of the ways I can reach this goal."
Twell's fellow Scot Robbie Simpson, who won bronze at this year's Commonwealth Games marathon, had hoped to feature in Valencia but will not take part following a recent back injury.
"Over the past few days I've been able to do a fair amount of cross-training," he said. "Hopefully I haven't lost too much fitness. But now I need to concentrate on getting back to my running."