Pix, sorry, but I agree with Shades on this one too. It is a hard decision to make though isn't it, because doing the speed stuff makes you feel like you are really working and means you can do a bit better in the shorter races. It is so important though to be able to do the endurance. I could comfortably run 20 miles tomorrow at 11.30-12 minute mile pace, but I can't get my 10k time back down to where it used to be. I have sacrificed the speed for the endurance and I'm sure it's worth it in the long run.
Jane - I'm sure you could run 20 miles tomorrow too (apart from being tired from Sunday!).