very tough crust

Friends,

Last night, I baked a few loaves of Pain au Levain w/ Whole Wheat Flour from Hamelman's Bread book. It has a wonderful tang that is on the verge of being "too much," which I really like. There's only one problem: the crust is quite tough. Unlike previous loaves, there isn't any "give" to the crust. When I cut into it, crumbs fly everywhere. It's almost difficult to eat.

Let me talk about the process, since there are a few things I did differently that might be a factor.

First, the formula calls for a stiff levain build made with 95% bread flour and 6.5 % rye flour. I decided to use 95% whole wheat flour to see how it would affect the flavor. In the final dough, I replaced an equal portion of the WW flour with bread flour so that the overall formula wouldn't change. 

Second, I used a 50/50 mixture of White Lilly Bread Flour (13% protein) and Pillsbury Unbleached All Purpose (9% protein). I did the math, and this closely approximates the 11.5% protein that Hamelman recommends (I didn't have any King Arthur AP on hand). I suspect that the BF used red spring wheat and the AP flour used a blend of soft and hard winter, but that's just conjecture. 

Third, I did the final fermentation for 20 hours in the refrigerator at 40 degrees. I typically do this for 6 hours, but the schedule worked out such that I needed 20. Hamelman says you can retard "up to" 18 hours at 42 degrees. I let the loaves sit covered at room temp while the oven heated.

Last, I baked for 55 minutes at 440 degrees. Hamelman suggests 40-45 minutes. I chose to bake longer because the crust was a little pale.

Based on this information, is there any reason the crust turned out so hard? I suspect that the extra baking time had something to do with it, but I have also read that lower protein AP flour gives a crispier crust. Or, perhaps the extra time in the refrigerator have dried out the loaves slightly? (I wrap the couche in plastic, but the towel had clearly absorbed some of the water from the dough). 

Thanks for reading through this. I'm looking forward to hearing what the collective wisdom of the internet has to say.

Eric