You have to train at the shape you’re in now, not where you want to be.
Exact. I want to run a 2:04 marathon. But I’m not going to train like someone who will run 2:04, I’m going to train like someone who ran 2:52 recently.
This is one of the reasons I said you should go ahead and run the Jingle Bell 10k on the 11th. Then we can use that race time to determine your current fitness level and set up some paces based on that.
Train. Race. Speak with other runners. Eventually you’ll understand things naturally without need to look much at books or calculator of any sort. All that stuff is good for guidance and has a large degree of uncertainty.
Most people put far too much effort in worrying about details of paces and distances, and not enough effort and dedication on training.
Train more and think a little less about it.