Leaden Legs, Lightened Legs

Now that half marathon training is over for the season, it begs The Question: How does one train for a 500k relay? Answer: I'm not sure.
So I thought I'd experiment by training like I'm running it. I've been told I'll be doing 4-6k two to three times a day for 3 days. So I've decided to try training by going twice a day for 4-6k.
This morning was my second run since my half marathon. I won't count last week's run, as it was mostly a "introduce myself to the running group" sort of run. I did a 3k loop in stifling heat and called it a night. Woof!
Today I felt I would be much more recovered. So day one of the 500k training has begun!
...And boy, did my legs feel like lead. I guess it didn't help I started this morning with a bang. And by "bang" I mean a really steep, tall hill right outside my door. The whole course felt like a hill, in truth. Up-down-up-down. And my poor lead legs were beat!
First run: Loop through the Botanical Gardens (morning run).
Distance 5k
Elevation gain total: 97m (this is the total elevation: I went up & down)
Total time: 31.00min.
Pace: Average pace was 6.17min/km. The fastest pace I hit was 4.05min/k (going DOWN a hill).
Boy did my legs feel like lead! But my fancy gadgety watch was certainly entertaining to use and gave me all this cool data. :)

Rocky Road Runners - Railroad/Old Bridge Loop.
This was the same 3k loop as last week, but I ran it twice this time. I was hoping to run it with other people but in the end I ran in a gap between the faster and slower group - too slow for fast and with no desire to go slower (I saw this as a training run more than a social run tonight). I was feeling a bit tired from this morning, but I really wanted to push myself and see how well I could do. The lack of hills made a difference! My legs felt much, much better this arvo than this morning.
Distance: 6.17k
Total time: 35.28min
Elevation gain: 17m (notice the difference this had on my time from this morning!)
Pace: Average - 5.45min, fastest - 4.27min.