If you are 70 defence and don't want to go any higher than that (which I'm assuming you don't), then make sure you have "Pure Mode" on. Or else your character will train defence! Then leave the attack style on attack so you can get attack exp from killing the monsters.
Also, at 17 dung you will start off with what seems like horrible exp per hour (2,3,4k etc.) but don't be discouraged. Dungeoneering is one of those skills that is slow and seems horrible at first, but the higher your level, the faster you will be able to accumulate exp. I have a character who is 86 dung and when I use Idungeon Pro with him, he gets anywhere between 20-30k p/h.