I have a few questions.
> Forgive me if they seem redundant or stupid ;-).
> 
No such thing.  
> Try as I might, no matter how long or how steep or how fast we go up a hill,
> I can't get one of my horses HR to get up to 200.
I agree with Tom about not trying heroically to get up to those higher 
HR readings.  It sounds like the numbers just don't fit your guy.  Maybe 
the most important thing we should all remember is that you can read 
books and research studies until you're blue in the face, but it will 
never take the place of Knowing Your Own Horse.  This is heresy coming 
from a dyed-in-the-wool Research Person, but experience is still the 
best teacher.  If you know your horse, you KNOW when he's working hard, 
goofing off or really struggling.  How many times have we all been 
or heard a rider tell a ride vet "he's just not right"?  Listen to what 
your horse is telling you and THEN try to apply all the technical and 
scientific knowledge as appropriate, not the other way around.
Every time I get too caught up in what one study said or another one 
"proved", I remember what my first advisor (who was truly a giant in his 
field and very much missed) told me when I started at Cal Poly, "Never 
forget that you're going to spend thousands and thousands of hours 
trying to learn what the horse already knew the day he was born."
Susan Evans
California State Polytechnic University