Check your pulse everyday when you wake up. It's a good way to be sure you're still alive but what you're looking to see is if it's 10% or more above normal for three days in a row. If it is you've likely been overdoing something and should ease up for a bit. If it's not you most likely are fine. I got this from Lydiard and I like it because feelings of fatigue can come and go and sometimes mean you're pushing your running too much and need to cut back but also could be a temporary thing that will go away as you adjust to the bigger load.
Another thing you might consider is to run twice on one day and then do very little the next. There was a Scotsman in the 60s who did 2:13 for the marathon and won the London to Brighton ultra, 53 miles, and said afterward that he'd run more miles that day than he usually did in a week. He also said he'd learnt the value of running every other day but also mentioned running three times a day. The value of the every other day thing was recovery.
So let's say you had a couple days when you got 12-13 miles in with two runs a day and third day when you got your 16 in. You're at 40 and if you did 2-3 miles on three or four other days (the Scotsman did that rather than take an entire day off) you'd be doing a lot more mileage than you are now and could still have day completely off if you want one.
As I've gone this far, I will tell you that long ago I read of a study done on female British marathon runners and their training trying to isolate the variable that best predicted fast racing. It turned out to be the TOTAL NUMBER of runs taken during the prep period, not the total miles though that was the second best predictor. If you do something along the lines of what I suggest you'll be covering both of those bases.
Of course I don't know what your life is like and the idea of running twice on a couple days may seem too much. It does usually require some discipline and adjustment to get yourself to that. But once you get into that routine it becomes automatic and much easier. And it's just an idea I'm tossing out here for you to consider. If it's going to make your life and running semi miserable it's probably not worthwhile. But as I said, I am kind of impressed with 3:52 on the miles you did and want to give you something to think about.