Credit: Mike Marsland/

Jennifer Aniston's best friend of all might be her treadmill.

The actress, 41, is the only 40-plus -- or 30-plus! -- star to chart on the Women's Health "Best Summer Bodies of 2010" list, and staying in such great shape isn't easy.

"It's simple," she told the Daily Mirror at the London launch of her eponymous perfume in London on Wednesday. "[I] run, work out every day. I do a lot of running -- exercise is so important."

The single actress added, "Plus it's all about balance. Work, exercise, diet, you need balance."

Aniston suggested that her fame helps keep her motivated, too."I am, perhaps, fortunate in that my career, the public awareness it brings, gives me a certain freedom in this way. But, of course, there are positives and negatives in any career, it's not all going to be perfect."

Indeed, a source on the Hawaiian set of her recently wrapped flick Just Go With It told Us that the 5-foot-6 actress is "very conscious of her body...She's obsessed with staying fit and young...[She] rarely misses a workout."