Thanks for all the replies and help. I did the workout(s) today and it went pretty well.
To answer the questions, I'm training for 10km with current PR of 31'3x and hoping to run under 31'00 in the spring.
The morning's workout was deceptively tough as the first 8km at 85% of 10km pace felt very easy and I ended up running much faster. This made the next 8km feel a good bit harder and probably made me a bit more tired than I should have been before the afternoon session.
I took about 3.5hrs between the end of the first workout (10:30am) to the beginning of the second (2pm). I had a recovery drink right after and a decent lunch. I'm not training for the marathon, so wasn't concerned with starving myself between workouts.
I ran the afternoon session alone and in the pouring rain and wind, so it was a bit rough. The first 8km rep was significantly slower than the morning's, but still under the target pace and on a tougher course/in the rain. The 1000s afterwards were supposed to be at 100-105% of 10km pace, so 3'01-10. These were a bit slower because of the rain and because I was pretty tired (3'12 down to about 3'05).
Overall, the workout was:
9am (nice):
3M warm up
8km @ 85%: 27'48
4'00 jog
8km @ 95%: 26'27
1M warm down
2pm (rain/wind):
2M warm up
8km @ 85%: 28'45
3'00 jog
8x1000m: 3'12, 3'10, 3'08, 3'09, 3'07, 3'05, 3'06, 3'07
2M warm down
Hope this is helpful to some other folks considering doing something like this. I'll add the caveat that I've been running 100-120MPW for about 18 months, so you definitely don't want to hop into something like this without a reasonable mileage base. We'll see how things go in a few weeks. Now, it's just time to recover.