You would think carrying around a million-dollar rock would be exercise enough.
"She looks fantastic from head to toe," Pasternak tells E! News. "Her arms are definitely leaner."
Pasternak currently has Duff on his Five Factor workout five days a week, but he says each session is never more than 45 minutes long. His training methods incorporate cardio, circuit training and weights.
The fitness guru adds that his program also includes a diet that consists of five small, healthy meals per day made with simple ingredients.
But it's not all about just looking great on your big day. Such an exercise regimen can also help calm any pre-wedding jitters.
"It's stress release and makes you feel more confident," Pasternak points out.
As for Duff's fiancé, Mike Comrie, he won't be looking too shabby himself when the couple says "I do": The NHL star is also a client of Pasternak's.
See more celebs getting into shape in our Sweatin' With the Stars gallery.