I've always had to drag myself kicking and screaming into speed work but I'm now following the Runners' World 5-week Cross Country programme from the October issue, which introduces 2 speedwork sessions per week.
With someone else (albeit a magazine) telling me what to do, I'm finding it easier to introduce speedwork into my training and not overdo (or underdo) it. I'm also using a heart rate monitor which tells me if I'm working hard enough (or too hard). I found the info on this website useful for getting the right working heart rates sorted out.
Yesterday it was 8 x 2 mins at 5k pace with 90 secs inbetween. Tougher than it sounds (I did it on a hilly course ... didn't have to either!).
I've only been doing the speedwork for 3 weeks but already am feeling so much faster and 'plod' a lot less when I run.