IF you raced a full effort, you could definitely do a lot faster with more even splits. 64/72 is a pretty big gap.
Were you comfortable at 400? If you have 60-second 400 ability and come through halfway of an 800 in 64, falling off to 72 would be a sign that your aerobic base and lactate tolerance are major limiting factors. (Don't neglect the speed, either. Your flat 400 speed is of course always a hard ceiling in terms of how fast you can get out in an 800, no matter how good your endurance.)
BUT: a solo time trial 800 is very hard for anyone mentally, and the fall-off in the 3rd 200 may not have been totally due to your fitness. What do you think? If you had that unsatisfied feeling afterward, like it was more of a workout than near a true race effort, you might be in much better shape than your TT indicates.
This is why I like off-distance TTs like 600s. An 800 is that much more daunting mentally and then you risk obsessing over time unnecessarily. I've read that Duane Solomon does a 700 TT pretty often, treating it like the first 700 of a race and thus getting a really good test without quite taking it all the way into the well.
Maybe time trial a 600, aiming for one second per 200 faster than your goal 800 pace. If that works out, you are pointed in a good direction.