So if you burn iron you're basically un-doing that process.
Iron might be feasible as an energy storage system. Article says it has low specific energy so I'd question if it could ever really be feasible for transporting energy or useful as a vehicle fuel (for ground vehicles let alone aerospace).
If new geosurvey technology revealed a bunch of giant veins of pure un-oxidized iron, or if we start replacing all our steel buildings with plastic or carbon nanotubes or something and producing so much metal scrap and recycled steel that it starts ending up in landfills? Then it might be worth extracting the energy from iron. But neither of those scenarios seems particularly likely.
Also I'd be worried about temperature issues. If your iron burning engine is going to be, well, running hot enough to burn iron, you probably shouldn't make it out of steel. You'd need a material that's more expensive and harder to work. It's doable but seems like it would cause annoying fabrication issues and increase costs.