Well, a strange thing happened on the way to the forum:
I was able to do the 3 miles at 10 mph, and it seemed easier than it did to do it at 9 mph!!!!!!!
I am not in substantially better shape, & I did not just drop 20 lbs.
I guess 9 mph is just too slow for me, and is somehow inefficient and artificially fatiguing, so actually required as much or more work than running at 10 mph.
The natural question, then, is why the heck was I running at 9 mph and not 10 mph or higher? The answer is that I'm not a distance guy, so I started out at 8 mph a while back, and have been increasing my speed in increments, leading to 9 mph.
Of course I did intervals at 15 mph, but never at 10 mph, and therefore I never realized that 10 mph is easier than 9 mph.
It is also totally counter-intuitive. When laboring at 9 mph, who the heck would RAISE the speed to 10 mph?
Heck, next time I'm going to try 10.5 or 11 mph.
Maybe that's the difference: when running freely, you can settle into whatever pace is comfortable, almost instantly. When speed is constrained on a treadmill and the response time is long, you don't "settle" into a good pace at all--you have to have some idea of what is a good pace for you to begin with, and set the treadmill accordingly.
This is probably no revelation at all to endurance runners, but to a hack like me, when it is zero fahrenheit outside and a treadmill is readily available, it is an epiphany.
BTW, regarding footfall and retarding of the belt: I burned out our True treadmill, which was the only one that went to 15mph. The thermal breaker now trips any time it gets going reasonably fast. Talk about a drag!