Duke's first Aspen Derby - Novice & Training

I was very happy with Duke's first derby, although I maybe shouldn't be.  We went novice and then training level.  His novice round was textbook - nothing at all to complain about.
I had to spend about 20 minutes studying the two courses to learn them on paper, and then during the course walk, I walked both of them.  They lines were clean and simple and direct, so that made it easier to remember both of them.  They had almost nothing, well, one line, in common (irony foreshadowing).
It took much longer to walk both than I thought, so I had to scramble to get on him and get down to warm up.  He was a little tense on the way down, and snorted his way around the indoor arena the first time.  He was shying at anything big and lumpy (a propane tank, a water tank), but not in an unbearable way.
I put him to work and he felt fine, but a little short.  He was fine yesterday, but accidentally had Friday off because of work.  We jumped each jump once, the cross rail, the vertical, and the oxer in warm up, and each was spot on, so I let him walk a few minutes and then we went and rode the course without even watching another rider.
Absolutely nothing to report - he jumped each fence like it was easy peasy.  It was a big log, left turn to another log, left turn around the outside of a line to a vertical, left turn to a roll top, right turn to a 2 stride combination with an oxer as the first fence, right turn to an oxer, left turn to a vertical, bending left (long) line to a roll top, and then left turn to a slightly skinny vertical butterfly.  Duke didn't bat an eye at any of it, and switched his own leads.
We had an hour to kill in between, so I walked him back to the trailer, then walked him around the property on the road.  I noticed on the very slight downhill on the trail that he kind of sat back and minced down, which seemed odd.  Then we walked around inside for a while, and he chewed and was relaxed.  Then we started to warm up.  He felt tight in the trot, and I tried smaller circles a couple of directions, and he might have seemed a little off to the right, but not enough that I could tell and I thought I was just making myself crazy.
So we cantered, and it was a pretty lethargic canter for him, and I again thought I could maybe feel something in the back, but wasn't sure.
So we started jumping, and he knocked the rail down on the vertical four times in a row.  I walked him, but couldn't feel anything, so we jumped the oxer and clobbered it, knocking the whole thing down.  Back to the vertical, and he showed it some respect, and then back to the oxer, and it was kind of puke jump, but he cleared it.  By that point, I couldn't decide if I should pull him out or just ride the course, since this is pretty unusual for him.  I decided not to do any more warm up fences, but to ride the course, thinking that we could have a show where he has worked hard in cross country and feels stiff and tired, and I wanted to know how he would ride stiff and tired.
The answer is, fine, except for the one line that he had already jumped.  That two stride combination with the oxer to start?  He half jumped it and then half just landed on it.  So we knocked the entire thing down, every single pole, every single standard.  He didn't seem off, so when we rode it again, he gave it some respect (which made the two stride tight), and then had a bit of steam for each fence, but didn't run off or get too strong after landing.  But he did, as soon as we jumped the last fence (which was a big table shared with prelim), walk immediately.
This course was a long combination to start, vertical to oxer, then a right turn and long canter over to a vertical, then a hard right to a slat table, hard left through two fences to the butterfly, left turn to a semi-roll back turn to a big round top, right turn to the oxer/vertical combination, right sweeping turn to a roll top, bending right line to a corner, long right turn but through a bunch of fences to a big wide table.
He did not spook as much at the big fences as I thought he might from last year, although he did look at them as we started both times, until I put him cantering and he realized he was showing.
He was good at the trailer, and good on his walk other than a few looks.
But the crapping out in between ... if we had been at home and he felt like that, I would have gotten off and given him the rest of the day off.  I gave him a gram of bute, his frozen boots on all four legs, his cooling gel, and he's in his back-on-track boots for the night, but I will be very curious what John says.  It felt like he just ran out of spring in his back legs and just couldn't push himself up and over the fences, not like he was being bad or lazy.  I hope his feet/legs aren't bothering him again.  We're at 3 months from the joint injection.
The good news is the fences did NOT look too hard.  The final table was a little big, but jumped fine.