Ermm for you, it's best to do 12km all the way at zone 2 without rest, if your goal is to run full 10km/16km. This should be your staple/go to run. No need to be fast, if you feel that you can slightly increase the pace to zone 2.5 (steady state run) during the run especially at the end, you can try doing that.
As for hard runs, I think you can still go for intervals as a separate run maybe once a week. With your hard run 6km timing, extrapolate the target race pace timing for your intervals and do a 5 x 1km and work up to 7-8 x 1km at the same pace or faster pace, whichever works best for you. This means you don't need to aim for so fast, something like 5.30 - 5.40/km is a good goal to aim for.
Or you can try lactate threshold, where you run the 6km at zone 3, at a pace of 10-15s/km slower than your actual 6km runs and slowly improve that timing across the weeks. But note that this is usually the less intense method and slower speed gains, but give more stability. You wouldn't downslide so much if you don't run because zone 3 builds better base than intervals.
8 x 888m intervals 100Plus Promenade w/ 1min rest in between (ASICS)
- First set a bit too fast since I paced with Singapore Sport School xmm, she was slightly ahead of me. But should be around the same pace since she started her run earlier.
- Second set, slow down since I realised not good to pace too ahead and can't finish the sets.
- Third set, felt the stitch about to come so I slowed down.
- Fourth set, asked to take a group photo after the run so rested more, this helped me to slowly lose the feel of the stitch.
- Sixth set, was actually able to run this set fast but decided to slow down and pace a fellow ASIC runner since I can tell he was struggling a lot.
- Seventh/Eighth set, just push through since nothing to lose
Thankfully xmm didn't end up lapping me even though I had to take group photo and slow down considerably a lot. Much better run than the FFRC one.