Nero would no doubt be fully aware that his vessel was not a warship. He'd prepare his ambushes accordingly. Remember his modus operandi: he kidnaps key personnel, obtains information from them, and uses that as a weapon. Those missiles must have been but part of his true arsenal, and the only part that was a conventional weapon as such (if that - they could have been mining tools, too).
Destroying the cadet fleet was a less impressive feat than keeping anybody on Vulcan or Earth from blasting the drill to bits with a sidearm or a pickup hovertruck mounting a Gatling phaser. Neither missiles nor red matter could have achieved that! Nero was a sneaky bastard who didn't need much in the way of conventional weapons because he had had twenty-five years to come up with a plan.
Indeed, the fight with the Kelvin
probably taught Nero that he should never engage in a straight-out fight with a starship, not without an ace or sixty in the sleeve.
Like max warp for the Enterprise from Earth to Vulcan taking 30 minutes to an hour in ST09 but 3 days for the refit ENT in TMP.
Why less than an hour? Kirk could have been out cold for much longer than that.
Not days, of course. But we could always argue that the three days in ST:TMP were a thing Scotty was lamenting
. That is, the refit and the adventure with V'Ger had made it impossible for the ship to maintain higher speeds for the next few days at least.