To have the game suggest doing it on hardened you had to score under like 23 seconds on the mockup to get Veteran as the suggested difficulty you had to score under 20 seconds (there is an achievement for this on 360). On MW2 it was under 30 seconds for the Veteran suggestion on the training mockup (and another achievement for this too). It's an interesting way of determining player skill and in theory, as you play up the levels you've learnt enough skills to be capable of surviving though you can always put it lower if you wish to. Likewise you can select veteran even without it suggesting it to you, it will however suggest that you will find it tough to complete the game and should do a lesser setting.