Hey I have a defence pure . It's a summoning tank now cause I'm going for 99 Defence,HP,Summoning, and Prayer and rest of stats at 1. Get it 60 defence. Then buy members if you want. Since you already got 40 Defence, you can stake. Or you can train the other stats.
It's kind of pointless to train Defence in F2P, as I too have a Defence pure. If I were you (don't know if you can afford membership or not), then I'd just keep trying to earn cash in F2P.