Making your training specific to the sport and events that you're training for will pay enormous dividends in improved results.
Two events over the last week have led me to think about the need to make your training specific, especially if you are an age-group athlete with limited time.
The first was a conversation I had with a friend who is training for criterium racing. His coach had been prescribing low cadence, high gear climbing intervals, apparently for strength development. Based on an assessment of the event demands of a criterium race, this would seem a waste of time: He's going to get good at pushing a big gear slowly (up a hill). Not much call for that on race day.
The second was an early morning ride I did over a hilly course, where I found that I simply couldn't ride it as I had done in the past. My very limited training has been criterium-focused and I haven't ridden hills in ages. My ability to ride hard at a high cadence on the flat with lots of corners and in a bunch for an hour to 90 minutes hasn't prepared me for riding hills and it would have been embarrassing had I been with anyone else!
The missing element in both of these cases is the specificity of training. For someone with limited time, you have to decide what's important and place your focus firmly on those things only.
The need to make your training specific pervades our sport. Here are a few examples...
At the most basic level, if you want to be good at a sport, you have to train that sport. You simply don't get really good at cycling by swimming or running. They're all essentially long duration aerobic activities, but use different movements, positions etc.
You need to structure your training so that it matches the physiological demands of your race. Running a marathon is different to running a 1500m track race. If nothing else, you would pace these differently, but you also have to keep in mind how important it might be to cover competitor moves, the need to change pace during the race and the importance of being able to sprint.
A 25-mile cycling time trial is vastly different to a 1-hour criterium race in terms of physiological demand. One is evenly paced for an hour, while the other is a series of short sprints with incomplete recovery over the same hour. The training needs are similarly, quite different.
Distilling it down yet further, you should consider seemingly simple differences like the surface on which you might be running. If your race is a marathon on the tarmac (asphalt), you need to do a lot of your training on the tarmac, contrary to the common advice to "save your legs by running off-road". In the same way, if you're training to run a trail marathon, the majority of your training should be on trails.
If your race is hilly, you need to train on hills. If it contains a lot of corners, you need to practise cornering. If it's a bunch race you need to train in groups, but if it's a solo race you need to spend a lot of time training alone.
If your race goes through the night, you need to do some training at night - mountain bikers quickly discover that seemingly easy trail sections become trickier at night because everything seems to come at you more quickly.
Swimming in the ocean is different to swimming in a lake. And both are vastly different to swimming in a pool. Wherever your race will be, you need to do as much training in that environment as possible.
I'll finish with one common mistake that cyclists and triathletes make, especially now that virtual reality trainers are so accessible. The only time that you should train solely on a turbo trainer is if your race will be on a turbo trainer.
While indoor trainers are awesome tools and I use them extensively to help clients to do sessions that tightly control intensity, they do not take account of all the environmental factors you encounter on the road or trail. Therefore, once again, you need to make sure that you ride outdoors over terrain that mirrors the conditions you will encounter.