Hope you don't mind if I chip in a bit stu.
Threshold sessions are good, and will improve both your speed and stamina, but it's important to build it slowly to get maximum benefits.
You could start off with maybe 5 x 400 with a 3 min break in-between.....warm up with a real slow jog for 10 mins, then run 400m( or whatever distance you think you can repeat the at the same pace x5, and either walk or slow jog in-between.
Now the trick is to not bollocks yourself by running too fast on the first couple of repeats, the science says you want to just dip slightly under your lactic threshold on each one but only a tiny bit(think of lactic threshold as when your legs start to feel heavy)
Like I say, start by doing something you think is quite easy, and keep doing it(sorry to disagree stu) twice a week is what I'm following and it works as long as anything else you do is really easy.
I've not long finished reading "the Norwegian singles method" (nope it's not a swinger's catalogue) and it really does make a lot of sense with the science behind it.