What I did to drop from 35:52 to 33:35 in one season:
1) Run 6 days a week at least, twice a day (6km in a.m. Mon-Fri)
2) run at least 60 miles per week
3) long runs of 15-16miles on Sunday at easy pace (7-7:10/mile)
4) Steady state run at HM pace on Monday (6-8miles) started at 6:20/mile for 10km in May and by August was doing 8miles at 5:45 pace..was not a hard workout but did the trick
5) Tuesday 8-12 reps of 400m @ 3000m (1:12-1:10) pace w/1:15 min rest
6) Wednesday easy (6km morning,10km afternoon)
7) Thursday repeats of 3 X 1km on road at 5km pace with 2min rest, unless a race on weekend...by August these had dropped from 3 X 1km in 3:20-:25 to 3:05-:10.
8) Friday: same as Wednesday
9) Saturday-no running, maybe long bike ride or nothing
Also dropped my 5000m time from 17:35-17:02-16:35-16:25-16:05 and HM from 1:22 to 1:17 (on a cold, windy and rainy day).