The short answer is quality training and patience. You're body will improve in it's own time...you can't rush it, or you will risk injury.
I've said it before on other threads, and I'll say it again...I use the hills.
I tried speed work for a good 18 months a few years ago. I would do good quality speed work and fartlek once or twice a week. The speed work entailed 8 to 10 x 800 mtr up 1 mile splits of 6 to 7 min mile pace (my average normal running pace was 9 min/miles back then). It was hard work but I stuck with it.
But to my dismay in races my speed didn't improve at all!
Then I discovered the benefits of hills.... and have never looked back.
I ditched the speed work (which I hated anyway) and replaced it with specific hill training. Now, that doesn't mean running a route with some hills in it, it means finding a good hill of about 200 to 400 metres, running hard up and jogging down. Do that as many times as you can, I think when I started hill training I was doing 4 or 5 reps and got up to 10.
The first year of my new hill training regime I got a 10k PB by 5 minutes, a half marathon PB by about 6 minutes and a marathon PB by over 10 minutes. I continued to improve and can now regularly run 6.30 to 7 min miles in races. And to think that that was my speed training pace...and it used to hurt!
I'm quite glad that I don't do speed training now, I dread to think what pace I would have to try
