What product or technique should I use to restore an Old Leather Duffel bag?

I'd advise you to either use a leather cleaner, like one would use on their other leather goods or buy a spray bottle of Murphy's Oil Soap, spray a generous amount onto a clean, white soft cloth. Clean well, then with a damp clean cloth, dry it well. Once it's dry, take another clean, soft cloth, & buff it up, using small circular motions. MOS has a veggie base, smells great, & won't harm your skin.

try Mothers leather conditioner, look for it at a motorcycle shop