There are probably too many variables to figure this out via the internet. Here goes anyway. For me, excess fatigue and soreness is usually the result of poor sleep. I'm the same age and also run 5/3/1. We incipient old farts aren't sleeping as well as we used to. I can control my workouts and my diet, but not sleep. Even if I go to bed at the right hour the quality sometimes just isn't there. It tends to run in streaks for a few days, too. If your schedule allows, I find a nap or two is the best remedy.