Most of the time when I meet hostiles or npcs become hostile, Combat mode is first activated when they have run up to me, but I'm trying to roleplay a physically weak but powerful mage, so I want to initiate turn based combat, before they are close to me. I know that there is the on screen button, but is there a hotkey?

Did you try the Space bar? I believe it's supposed to toggle the real-time/turn-based combat.

The game is set to turn based combat, the problem is that combat mode is first triggered when the enemy tries to hit me, which is when he is close to me, he will run real time down to my character, upon which turnbased combat mode will be activated, only now I don't have an advantage of being far away.

Thanks!