Hi pan-man,
one of the things I did this year to achieve my 1/2M pb (1.34.20)was to run at 1/2M pace for at least one run a week, starting from 5 miles and building up to 10. I run about the same number of miles per week as you for both 1/2 and full. My marathon time is slower than yours, but I hope to get a sub 3.30 next LFM.