Surely if you're going to out-walk your prey to death you need to make
sure it's not galloping out of sight - any objections to ambush + bite
to the hips to severely cripple it, then walk it to death?
Yes.
If it were picking on animals it's own size, it would be as fast or
faster than them - they couldn't gallop out of sight.
And where's this mega-tall, long-tailed monster supposed to be hiding?
They'd need a lot of large and thick bushes! Also, duckbills and
ceratopsians probably had better acceleration than /rex/, because of
their shorter limbs and lower CG -- rexy would have to make sure they
came _damn_ close before trying to bite 'em.
So, I ask, what does it look like /T. rex/ could do better than it's
prey? Long-distance, efficient walking is my guess. I don't know the
energetics of the situation, but shouldn't think a walk of a few miles
would be a huge problem for a healthy /rex/, and it would only need a
small margin on its prey to catch up on them.