Kim, you've described my guy, Cheers, whom I competed for 13 years.
Although I never did 100's with him, we would often do two 50's in a
weekend. There is not a word in any language to describe the way he
lugged: my arms would be shaking with exhaustion after the first loop. He,
too, broke a few martingales. I would often be in tears on the first loop
because the thought of dealing with him for 50 miles was so overwhelming.
Any time someone passed us, or he could see someone ahead of him he would surge
ahead. On the other hand, I NEVER had to worry about him eating or
drinking. If he didn't, I knew he didn't need to. When he wanted to
drink he would come to a screaming halt (usually with me doing the screaming) at
whatever water source was handy, and almost pull me off putting his head down to
drink. When we got near camp, he would settle down to a nice big walk, his
ears forward and the sweetest _expression_ on his face....no one ever believed me
when I'd tell them what a s**t he was, until they happened to ride with
me! In all the years I competed him, he never did learn to pace
himself. However, like you, I grew to love him, and accepted the
limitations brought on by his behavior. Even though the horse I compete now is
soooo much easier on me, I would give anything to have Cheers back out
there! BTW, he also had a big fat tongue, and I found a french snaffle
seemed to lay in his mouth much easier.