It would help also to start some of the dough the night before with a preferment. The night before, combine a third (or so) of the flour and water and a tiny bit of the yeast (like 1/4 tsp), knead it and let it rest for a couple hours then refrigerate it. The next day, take the dough out for hour or two to take off the chill and combine with the rest of the ingredients.
I've never made 100% whole grain bread, but when I'm using whole grain with bread flour, I also knead for a good 15-20 minutes. And if you're making sandwich bread, lower the temperature would be in order.