You have eight weeks or more left for your build up, and if you are already doing close to 20km now, that should give you enough time to do a combination of:
- increase your distance
- increase your load
Everyone will have a preferred buildup pattern, so find one that suits you. I work full-time, so I schedule my longer buildup walks for weekends, and only do one long walk a week. I try and increase these walks by 30 min or so each weekend over the final weeks of my buildup. Even if you only made this increase every other weekend, you would still be adding a couple of hours or 10km or so, getting out to 30km before you left.
On the capacity side, I did some rough hill walking with a heavier day pack (25li and up to 8kg or 17lb) followed by a couple of slower multi-day walks with a hiking pack (65li, 16-20kg) where I was sleeping out. You shouldn't be contemplating this for the Camino itself, but you might during your preparation!
I will be doing something similar as preparation for walking St Olavs Way next year, where it appears one needs to be self sufficient (less perhaps shelter) for up to five days at one stage of this pilgrimage.
DougF