do u have to actually hit something with your desruction magic for it to go up? or is it just enough that you can produce fireballs etc and then your level goes up from you keep fireing them anywhere?

Nope it has to be a valid target. Just go out into the wilderness with no weapons equiped and use only magic.

EDIT: Something sorta related, if you want an easy way to level up on Destruction and Conjuration, get a low level destruction spell and a summon zombie or summon skelton spell and summon the zombie then use the destruction spell to kill him, it will level up both types. Note you have to be level 25 in conjuration to summon the zombies.