What is the fastest way to get your defence level up when you're a mage pure? (Runescape)?

Okay, I have a mage pure on Runescape with 35 defence, 73 magic and no attack, strength or archery. I need to get 40 defence so I can wear Sacred Clay armour. Please help me with my question and I will be very appreciated! Ps. Please don't bag on me for liking Runescape, I think it is a fun game and a great pass-time...