Training is basically stressing the body enough so that when the stressful period is completed and the athlete refuels and rests, the body responds to the stress by rebuilding a little stronger. The stress can be an increase in distance covered which, when recovered from, will improve the athletes endurance, or ability to go further next time.
The stress applied to the body may be asking the athlete to go faster than before over a short distance, followed by a rest/recovery period. This is often called “speed work”. The best results are made when the increases in intensity are very gradual, allowing the body time to respond each week, rising to a higher level of fitness. It’s important to not increase the intensity too quickly and cause damage to the athlete, which is likely to occur if the stress is greater than the athlete can absorb. None of this is new, it’s how it’s been done for 100yrs , but even in this age of information overload, a lot ignore these simple rules.
An area of fitness often overlooked in this age of the internet, is technique development. There’s not much point in repeating poor technique over and over, stressing the body expecting a positive outcome. So an important part of coaching is making sure that what the athletes are practising is the best possible technique. Practising drills in each sport can be as productive as applying lots of stress but with poor technique. Rehearsing the drills prints in a behaviour pattern which the mind replays on race day when the athlete is under stress and not thinking clearly.