you probably will need all that fartlek etc. i did 3.20 at rotterdam last year, which was bang on target, and i attribute that to the many track pace sessions that i had done in the previous 16 weeks, plus long, long fartleks with friends on Thursdays and shorter hill or speed work on Tuesdays. plus the obligatory long run on Sundays. all i can say is that the faster work worked and i don't think i'd have got anywhere near my posted time on long steady runs.
good luck, and enjoy yourself