Mark, welcome to the sport.
I would guess that greater than 99% of the bowls you see have not been turned "whole log" as you queried, but rather from sections of the log, commonly referred to as face grain orientation.
If you stick the whole log on the lathe, the grain runs parallel to the lathe bed and is referred to as "spindle orientation", because that orientation would offer the best strength for spindle turning.
However, it is quite difficult for bowl turning, and especially for new turners. Cutting end grain is very difficult - even with a chainsaw. Try running your saw down through a log standing on its end - really tough, vs on its side. Your bowl gouges will react the same way. Even when it can be accomplished special techniques (and sometimes tools) are necessary so that you cut "downhill" with the grain vs an uphill struggle. Hollowing the interior of an end/spindle grain bowl can be a very disheartening experience for a new turner.
Sometimes I do turn bowls end grain, but it is rare and only when I have a special figure that I want to present in a special log, like this very striking ambrosia maple log....
If that were turned normal/face grain, the streaks would go across the bowl instead of radiating out from the center.
What most people would do with your logs is cut them up in halves, removing the center pith, and then turn bowls from the half sides. It is easier on you, lots easier, and you'll enjoy turning more. I'd suggest that when you get more proficient, you experiment with this type of bowl turning, but for starters use the traditional grain orientation.
Also - no 12" thick black walnut log is dry. It will reach an equillibrium moisture point where it isn't losing anymore, but the interior will remain wet as far as bowl turners are concerned. It will remain wet enough that when you turn it, you will experience the normal distortion and warping that occurs with turning green wood. It is part of the fun, and learning.
