Indeed, fits perfectly the requirement "Anyone have good suggestions for running trails in kowloon and new territories?" :-)
OP - Mac Lehose Stage 5 (Starting Marker 101 - Shat Tin Pass) - bit of stairs to start (you can park near Sha Tin pass) but then mostly flat and shaded. you can go all the way to marker 108 (so before Beacon hill) and return - that's about 6km
Can also do the same with Wilson trail stage 5 - start at the same place as the above. mostly shaded. You can do a back and forth as well.
Also Wilson stage 3 from Lam Tin (can park in Lam Tim, then up the road and undulating trail mostly).