Topolev. Good analysis. But again, my example is not hypothetical. This is my specific ride. What drained my energy was taking a heavy bike (and wheels and tires) up rolling hills. I wasn't drained by the aero drag. I can do 100 miles at 15mph on flat ground quite easily on that bike, but the hills will drain me sooner or later on a heavy bike.
If we are going to go down the theoretical rabbit hole - again you are right - at 15mph I'm losing maybe 30 watts if I sit upright. Specifically 160 watts vs 130 watts But again I wasn't sitting upright. my specific bikes had the same amount of tuck and I wasn't using "modern wide bars." Realistically, most calculations show the friction drag graph and the aero drag graph crossing over between 10 and 15mph. In my life - Aero just isn't a big deal at speeds that give me that apparent wind speed. at <18mph I'm looking for any aero advantage I can get. But that's just me. ;-) Again, I'm just sharing my experience. YMMV. ;-)
Back to the OP. Yeah - the weigh made a difference in my endurance. Its been said that wheel weight is twice the penalty as frame weight and having heavier wheels and tires exacerbated the problem on the rolling hills on my route.
Alternatively, if I'm doing a flat ride with no acceleration (on that mountain bike, doing a high speed time trial) - weight doesn't mean anything.