I think in a way it's good to be underprepared. You've got a whole month to get the hang of this camino thing. I loved the feeling of getting fitter each day I walked, at least that's my excuse for not training
. I feel I'd have lost something of the experience if it was easy from the start.
Spain has shops (though not so many big ones until you hit Pamplona), so any kit you decide you need is only three days away.
Get your boots, socks and pack worked out, everything else can be fixed along the way. Just one practical note- cash is king on the camino so make sure you always have some, it's easy to go three or four days without seeing an ATM.