I'll give another vote for doubles.
I feel like doubles are always a good idea if you think you're serious about becoming a faster runner. If you don't have time because you have other priorities, that's fine, but don't think that they're not a good idea. They just make people better runners.
The thing with doubles, though, is to not really change what you're doing with your primary training run. Just run whatever you think is your optimal training for that one run, and then for your secondary run just throw in whatever you can that doesn't take away from any other important runs you have coming up.
If you're just starting with doubles, that might just mean that your secondary run would just be a 15-20 minute jog. That's OK. Don't let anyone tell you that you need to run for a certain length of time, or at a certain pace for it to be worthwhile. I've seen too many people make huge improvements shortly after adding in 5 extra 15 minute jogs per week.
I don't know the mechanisms, but they work.
As you get used to them, you'll probably find that you can lengthen your secondary runs, and maybe run them faster. Great. Don't let them take away from whatever your optimal training for your primary run is though. It took me a couple of years of doubles to feel like I could lengthen them to 30+ minutes, but once they did I soon noticed another good jump in my racing ability--particularly over 5k and longer distances.
Once you're _really_ used to them, you might be able to add in some more fancy stuff with mini workouts sometimes, or those "special blocks" Canova and Arcelli write about in their IAAF booklet, but don't worry about that for now.
If you want to get faster just start getting in more runs whenever you can. Be sensible with your body, and make sure you get whatever your optimal training on singles would be, but just get in those extra jogs whenever possible, and you'll get faster soon.
Some people seem to get benefits within just a couple of weeks, others seem to take a couple of months to notice a difference, and I've even known a few who just couldn't get a benefit from it--though usually that was because they just were such a mess in the rest of their lives they couldn't schedule the rest of their day properly even if they theoretically had the time.