Singer-actress Jessica Simpson is walking six miles a day on treadmill to look slim on her wedding.

Simpson will marry Eric Johnson in July and she is doing all she can to stay slim, reports showbizspy.com.

`She's incredibly proud of the way she's stayed on track,` a source told Life & Style magazine.

Simpson even spends plenty of time on her treadmill.

`She'll watch TV on it, talk on the phone. Jess probably spends half the day there. Jessica has gotten her best body ever. She truly feels like she's in better shape than she was in her early 20s,` the source added.