I must admit my understanding of how maiden recorded their past 3 albums was totally wrong. When Steve said we put ourselves under pressure and recorded the album in 6 weeks, I thought they go into the studio mostly blank, with rough outlines of songs and then jam and record whatever that comes out immediately. I thought that is what Steve meant by capturing the essence of a live/organic sound. I believed it even more so when i first heard TFF because a lot of the mid instrumental sections did sound like a prog/blues jam session to me.
However, I don't think even a band as talented and experienced as maiden, could possibly bang out an album out of nothing in 6 weeks. Definitely not something as intricate and epic as Book of Souls. A more accurate picture (in my view) is that the band members have learnt to go into the studio with almost fully developed demos and tons of extra riffs/harmonies that could fit into any of the instrumental section of the epics. This is pretty much confirmed by Adrian's interview where he said he had demo tapes for 4 songs for Senjutsu. I am pretty sure the rest of the band probably does the same. So i assume what they do is basically rehearse and polish/embellish things when they are in the studio and then record it in a live setting in 6 weeks.