whole wheat sourdough bread

I've finally gotten brave enough to make my friend Jena's sourdough bread. Oh my goodness, it is SO good and SO easy....seriously....EASY! The hardest part is just thinking ahead enough to think about timing and making sure that I wont need to get up in the middle of the night to knead or bake the bread. I have had pretty good luck with letting the dough rise for as long a time as possible. I've been making this bread at least once a week...it makes DELICIOUS french toast and sandwiches! YUM!
I decided last week to attempt to make it with whole wheat flour.
When it says to use 6 cups bread flour I substituted 3 cups all purpose flour and 3 cups whole wheat flour. (I felt the bread may have been a bit crumbly, so next time I will try 3 cups bread flour with 3 cups whole wheat.) It did not rise very quickly, in hindsight I think it would need to rise longer. I also followed a helpful hint I had heard about letting the dough "proof" in a warm oven to help it to rise....start an oven preheating to 400 degrees...but only let it preheat for 1 minute. then put the loaf pans full of dough in and let them rise.
Instead ofdividing the risen dough into 3 loaves, I think I would only do 2 next time. This version of the break tastes a lot like a wheat bread my mom likes to get from Great Harvest. VERY YUMMY! I might just start alternating the different versions of this sourdough bread recipe.