When Natalie Dormer received word that she would be playing the warrior Cressida in the fast-action franchise "The Hunger Games," she immediately decided to up her running game

"In Mockingjay – Part 2, I'm basically running around in heavy armor while carrying a semiautomatic rifle," she said in the new cover story for SELF magazine. "I don't know how I would have kept up with [my costars] Liam Hemsworth or Sam Claflin if I hadn't been in the best cardiovascular shape of my life." 

Dormer, familiar to "Game of Thrones" fans as the beloved Margaery Tyrell, said the first thing she did was sign up for the 2014 London Marathon. That goal in mind, she then decided to skip the use of a personal trainer and instead embrace the tried and true method of pounding the pavement during her off-time. What she didn't expect were the 12-hour shooting days for "Mockingjay," which forced her to report to the set before dawn - and find time to workout at night. 

“I actually look back on it now and think, F—ing hell, how did I do that?” she says.

Dormer, 33, said she survived with a combination of napping, yoga, a clean diet and ample amounts of green juice in the early morning hours. 

“When you have early calls, you don’t necessarily want to eat first thing in the morning,” she said. “But you have such long days that if you don’t, you really fall down from it. So, juicing before and eating later is, for me, a good nutritional compromise.”

Dormer, who ended up finishing the London Marathon in an impressive three hours and 50 minutes, hopes to complete the New York City Marathon later this year. 

"This will be my second marathon, but my first experience taking on the five boroughs of New York City," she writes on her fundraising page for the charity Team for Kids. "I’m going to give it my all, joining more than 50,000 runners from around the world, as we come together to take on the race of a lifetime. I’m so excited for this challenge and can’t wait to help inspire others by spreading the message of how important fitness and physical activity is, starting from a young age."

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Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.

'Game of Thrones' star Natalie Dormer's killer workout
Actress reveals how she ramped up her cardiovascular health with no trainer and plenty of green juice.