"Minimum Effective Dose" vs "Maximum Tolerated Dose"
Ambitious athletes often think of training in terms of how much their body can handle- in both volume and intensity. A better approach may be to think of training in terms of finding the "minimum effective dose" (MED) and your "maximum tolerated dose" (MTD).
Athletes new to a sport may find that building their fitness to a point in which they can complete the event at any pace may be all they can handle. In this case, the minimum effective dose equals their maximum tolerated dose.
More experienced and elite athletes by no means want their fitness to be at the minimum necessary to complete their event. For example, an experienced runner training for a half marathon will often run much further than 13 miles in training. They will run many miles slower than race pace, and some miles faster than race pace.
Triathletes often forget this point. If you're competing for a 70.3 race, completing rides longer than 56 miles in training will help guarantee you cycling stamina is beyond the minimum effective dose point. Check your decoupling from a long ride or run to confirm you've built up enough stamina for your event.
The best athletes have a great sense of their personal MTD in both volume and intensity. They also acknowledge that it may vary based on the time of the year, and simply over time.
My experience indicates that the MED in terms of volume is higher than the average athlete expects (it takes more time and miles than expected to become very good). My experience has also shown that the MTD in terms of intensity is less than the average athlete thinks. Like salt on food, a little training intensity goes a long way. Too much, and you'll ruin the meal, too little, and it won't be as good as it can be.