I would *not* bleach it. You'd most likely get a crap result. Don't know if they're tough enough to throw in the washer with some strong detergent. That'd probably help.
Personally, I would hand wash it with a lot of detergent a few times, crumple it up and rough it up to get a worn look. Scratch it/sand it with some rocks or sandpaper (very lightly and carefully, going for the corners and such for a naturally worn look), set it outside someplace safe in the sun for a week. And then I'd finish it off with a thorough dirtying. You could buy "Fuller's Earth," but I've found a great substitute for that. It's called "earth." It's free and it's right out you backyard. (It's everywhere!) When I really want to dirty something up, I follow one of two courses. I make mud and smear it all over, let it dry, and then just brush it off. Or I wet down the item, smear dirt all over, let it dry, and then brush it off. Just rubbing dirt on something dry doesn't tend to make it that dirty.
The dirt will both make the bag look more worn, and it will lighten it.
I highly suggest taking a before pic. (And posting it here with an after once you're done and satisfied.)