There can be this big misunderstanding that we only use one type of energy system for all of our activities and that it does not matter what type of training you do just as along as you do a lot you'll get in shape. However that could not be further from the truth as our body works off of three different energy systems, one is immediate and is used for explosive rapid movements another one that copes with the demands of relatively high energy output but for short amount times and then the third is a lower output of work but is able last longer duration of time.
The first energy system is the phosphagen system it uses creatine phosphate and has a very rapid rate of ATP production. What's great about the system is that it works immediately and helps with short term high intensity activities lasting about 1 to 30 seconds It also is great in the fact that it produces ATP at a very fast rate. Picture Usain Bolt running down the track 100m and the primary energy system that he's using for that activity would the phosphagen system. However the downside to this energy system is that in using creatine phosphate and ATP, those resources are limited because there small amounts stored in the muscles which is why you can only sustain explosive movements for a very short period.
Which leads us for our next energy system the Anaerobic Glycolysis aslo known as the Lactic Acid system that doesn't require oxygen and uses the energy contained in glucose for the formation of ATP this system it can work as an intermediate and pathway between the phosphagen and the aerobic systems. It can also produce ATP quite quickly for use during activities that require large burst for longer periods of 30 seconds to up to 3 minutes, the example here would a 200m swimmer.
In the final system that is deemed to used the most would be the oxidative system also known as aerobic glycolysis it requires oxygen to produce ATP and this occurs in the mitochondria of the cells and is used to sustain activities of longer duration so like running Marathon or going for a really long bike ride. The downside to this energy system is that it has a very slow rate of ATP production, but when this energy system is properly trained it can help a person keep a steady pace without stopping.
Now that you know about all three it's important to know that they work entirely in sync with one another with the goal of being efficient and having enough ATP production for the muscles to contract. The perfect example of an athlete using of all three systems to their max perfectly would be Cristiano Ronaldo during a soccer match. His ability to react and be explosive for the whole match is nothing less an stellar.
Now how does a person get to be efficient like Ronaldo, in the activity they partake in. Well first, the focus should be centered on the activity whether it be weight lifting, sprints or long runs a person should focus on training that best suites those activities. For example doing a 10 mile run is not going to be the most efficient type of training for football player nor do you want to be doing nothing but Olympic lifts as a Marathon runner.
The moral of the story like anything else is you have to have some balance between the three energy systems but put an emphasis on the energy system that best helps your activity or sport.