If you were to take a specific event and a specific athlete, you can get some idea of the things it takes to specialize a body to accomplish one specific task.
Let's take Kenenisa Bekele and the 10k as an example. He will likely start with a long period of what they call "base" building in which he goes on very long and relatively relaxed runs. He might be running anywhere from 80-130 or more miles per week in this phase without too many harder efforts. He might be doing it at altitude as well. This is mostly to build up the amount of red blood cells and the ability of the body to process the oxygen in them and transport them to the muscles effectively.
Then he will begin to add in faster efforts. These will help him be able to withstand the incredible stresses on the body during a fast 10k. He will work on his body's ability to cyle lactic acid and to handle the physical pounding of running sub 4:30 miles for a 6.2 mile race.
Of course he will also be racing, probably worrying about what he eats, making sure to get lots of sleep, getting massage to work out his muscles and make sure he stays loose, perhaps adding in some weight training or flexibility exercises, but almost everything is geared towards simply covering a lot of ground very quickly.
The specific answer to this depends very much on the individual event involved. Different events need bodies that are "sculpted" in different ways. Different events also need different biomechanics.
In general, these things have been done by:
- Better nutrition.
- Better techniques for weight-lifting and working out.
- Very good high speed video equipment and computers. With this type of equipment, an athlete's movements can be broken down in a very accurate and detailed way. Then coaches can look at the exact movements that, for example, cause an athlete to get over a hurdle most quickly.
The first two of these help with the "sculpting" while the last helps with physics and biomechanics.