Jennifer Aniston keeps fit by running around a horse track.

The 'Bounty' actress amazed crew on the set of the new movie, which also stars Gerard Butler, by getting up at 5am and running laps around New Jersey's Monmouth Park racecourse, which is near where she has been shooting on location.

A source said: "She showed up every morning at 5am to run laps on the giant oval for a solid hour before cameras rolled.

"She's really disciplined about keeping fit. She'd get there before everyone else and zip around that huge track like a winner!"

The 40-year-old actress' main exercise is running and she prefers it to other fitness routines because it keeps her thighs and calves toned.

This is not the first time details of Jennifer's early morning exercise schedule have been revealed.

Earlier this year, it was claimed she was getting up at 3am to ensure she still had time to work out and stick to her hectic shooting schedule for 'The Bounty'.

A source said at the time: "Jennifer has had a totally overhaul for this film. She wants to be seriously fit, yet remain curvy.

"There are a lot of early starts on set, so she has had to get up at 3am to get two hours in the gym before getting ready for filming."