HappySlug
Sounds like you are a prime candidate for fartlek training. The word apparently means "speedplay" and is basically the best way of doing speedwork without having to worry about measurements or stopwatches.
An example would be to warm up for say, 10 minutes, then increase your pace slightly for the distance between 2 lampposts, drop back to your warm up pace for 2 lampposts, increase pace again (either to the same as before, or slightly faster), drop again and so on. The variations are endless (I'm sure many forumites have their own methods) and the short bursts followed by short recoveries is pretty much the same idea as a regular speed session, just not timed or measured.
You could build a session from the speeds you know you can do - ie warm up, 2 lampposts at 1/2m pace, 2 lampposts jog, 2 lampposts 10k pace, 2 lampposts jog, 2 lampposts at a pace that you could only possibly run 4 lampposts at, 2 jog, 2 at a pace you could only run 2 lampposts at, 2 jog, 2 at 4 lamppost pace, 2 jog, 2 10k pace, 2 jog, 2 1/2m pace, 2 jog, then cool down.
The recovery at a jog, rather than stopping or walking is very good for giving you confidence in a race. I always go out too fast in the first mile of a race, but knowing that I can recover while still running helps me through miles 2 and 3, by which time I will have settled into a comfortable pace.
Hope this helps.