The Being Human actress plays Shania, a girl from a London estate whose talent leads to her becoming a member of Britain's sprint relay team.
"I used to be sporty and played netball and football but this was before the boobs and hips arrived and slowed me down," Lenora told the Daily Mail.
"I was amazed to get the part, but then they sent me on a really tough six-week fitness regime to get me into proper shape," she continued.
"I jumped at the chance but had no idea just how intense it would be."
Despite a strict cardio and strength-building regime, which included 1,000 sit-ups each day and training alongside Britain's actual Olympic hopefuls, the 27-year-old actress picked up an injury during production.
"On the first day of filming the running, I suffered two stress fractures in my ankles," she explained.
"I was utterly devastated and felt I'd let everyone down. I'd only been training six weeks so I can imagine how real athletes must feel when this happens after four years' training."
Crichlow's injury was eventually masked using spikes, a sponge surface and camera trickery, with the actress conceding in the end it was a "very clever edit job".
"In real competition real athletes have none of this backup," she added. "Now I have so much respect for them. I'll be watching the Olympics with different eyes."
Fast Girls is out in cinemas now.
Watch Lenora Crichlow and her co-stars talk to Digital Spy about Fast Girls below: