Are 5k and 3 mile races the same thing? The answer is yes and no. Yes because a runner who can do 5k can do 3 miles. No because 5k equals 3.10615569 miles and in todays sports world where a thousandth of a second can mean the difference between victory and defeat, the extra hundred yards can make a lot of difference. Its like another 100 meters sprint added on to the 5k race.
A 5k race is all about pacing yourself as a runner. You need to know when to conserve energy, when to use short bursts of it during the course of the races and when to make the final push. The better the runner you are, the more finely you will be able to judge when to do what. And if you are a good enough runner to be able to define your race strategy in meters, the extra hundred can make all the difference.
Measuring your race accurately down to a few meters is something all professional runners do. But, to almost the same degree, so to regular amateur runners. If you are an amateur runner who has been competing in 5k races regularly, you may not consciously know at what distance to hold back and when to push, but your experience will inform your subconscious mind when the time comes to change your pace. And if you are putting everything you have into the race, your last burst will be timed to use up all your reserves of energy as you cross the 5000 meter line. But what happens if you suddenly find you have another 101 meters to go? Sure your body can do it. Its not vary far. But how fast can you do it?
If you are planning on a final burst over the last few hundred meters, your body, both physically and psychologically, has been primed to use up every last bit of energy it has over that distance. Thats why you often final people who are running comfortably for most of the race suddenly are barely able to stand after crossing the finishing line. The extra 101 meters required for a 3 mile race could mean staggering through the extra distance for those not prepared for it.
The best way to be prepared for both the 5k and 3 mile races is to run 3 miles during your training. This will condition your body to the distance. When you have a race planned, spend a few days before the race running exactly the 5k distance. Mark off the last 500 meters of your practice run and start your closing burst from here, or where ever you normally do it from. When preparing for a 3 mile race, do exactly the same thing for the last 500 yards. This will allow you body to adjust to the small but important difference in the distance.
100 meters many not be much but have you seen how much a sprinter puts in over that distance and how drained he is by the end of it?