First loaf of a double batch was rubbery

I Am brand new to making sourdough bread. I’ve been having a lot of success using my discard, but my first attempt at actually making a loaf was miserable. It seems like it took all day long to get to the baking stage. I was using the recipe for country loaf from Tartan Bread, which has a 75% hydration rate.
This recipe makes two loaves. I baked my first loaf last night, and the flavor was okay the crust was beautiful, but while it did rise a bit, it didn’t rise as much as I would have liked, and it was rubbery.
When I did a little research it was pretty obvious that I had not let it ferment in the bulk rise long enough. Now I have an entire another loaf of dough that is basically ready to put in the oven, but I don’t want another loaf of rubber!
Is there a way to fix this dough to still get a good loaf, or what can I do with it that the rubbery texture won’t be an issue?

You’ve done a good job diagnosing under-fermentation. Sourdough processes can indeed take all day or even longer. Recipes that call for 3-4 hours for the first rise generally require a warm environment and/or a super-lively starter – not really worth the effort to feed that often and watch it that closely in my opinion unless you’re baking every day. The end result loaf can be just as beautiful with a slow sleepy starter as long as you let the dough get to where it needs to go before moving to the next step. See this recipe for more info on that:

Here is an article that addresses the time issues with sourdough – how to live your life and bake regularly, not just on the weekend:

As for your second loaf, you could reshape it and proof it for a couple more hours at room temp (RT) before baking. Between the extra time in the refrigerator and the extra time at RT, it should get to where it needs to be. The reshaping is simply to redistribute the air and get that shaping tension back. It would probably be fine not to reshape it too.