I am a big proponent of doing a lot of short fast work on the run. By short, I mean 30 seconds to 45 seconds. This helps to significantly improve mechanical efficiency as well as running speed and turnover. Another big plus is the ability to get in much more quality time at a fast pace without placing too much stress on the body.
However, when it comes to sustained speed, there are some important principles you need to stick to if you want to see all the hard work come out on race day. Keep in mind, I am speaking in the context of an Ironman or Half Ironman.
Here are a few thoughts: