Two things I forgot to mention earlier.
First, I stretched Saga both before and after my ride. His right hind is definitely harder to stretch than his left hind, and he did not like me poking on either side of his tail to get him to lift his back. But we did it anyway and will continue to do so. Let me say that stretching a horse is HARD! But hopefully I will get better at it with time and practice.
Secondly, on the walk back to the barn, we went across a little streamlet (it's maybe all of a foot wide and 6 inches deep) that appears on the barn property whenever we have a heavy rainfall. The first time we went up to it, he looked at it pretty carefully but never stopped, and then LEAPED over it. The second and third times were similarly large leaps, but with much less hesitation. I ended up in an inglorious heap on Saga's neck (this does not bode well for XC schooling next Saturday, but maybe something miraculous will happen between now and then). Red, of course, walked right through it, since jumping it would have taken FAR too much energy.