I believe the three most important principles of training are 1) specificity, 2) specificity, and 3) specificity. I'll give you an example.
I've been coaching Ralph for 5 seasons now. Great guy and a very good athlete. He is both a triathlete (Olympic distance) and a bike road racer. During this time he's been successful in both sports qualifying for Triathlon Worlds several times with one top 5 at Nationals, and he's been a contender in his cycling age category (50+ and now 55+) with top 10s at his state road championships.
For the past four seasons I had told him that he would be a much better cyclist if he quit doing triathlons or a much better triathlete if he quit bike racing. While there is an obvious overlap between the two sports, they are not the same. There are many differences in how to train for these sports. The outcomes in bike races are largely determined by anaerobic endurance efforts lasting two to four minutes during key episodes such as on hills and with cross wind. The outcomes may also be dependent on sprinting ability. Neither of these, however, is critical to the outcomes in triathlons. Here, with few exceptions, the most important limiter is muscular endurance. The successful triathlete has the ability to produce a moderately high power output but hold it for a long time. Just the opposite for cycling. There success is dependent on very high power for very short (relative to triathlon) periods of time. If nothing else, any time you are training to be good at one of these abilities (anaerobic endurance or muscular endurance) you are not training the other one. So something is lost in training.
You can't train to be good at everything at all times. This is the principle of specificity. Basically, this principle says that if you want to be good at something you must exactly and precisely train for its unique demands.
Ralph didn't doubt my explanation of this for the past four years. He certainly understood it intellectually. But emotionally he wasn't ready to give up either sport. He loves both of them with a passion. Then something happened this year which changed everything and gives us a good lesson in specificity.
Ralph's cycling and running came along quite well this winter. In fact, his running appeared to be ahead of previous years. Then it happened in February. Ralph's Achilles began to act up. It wasn't too bad at first but it refused to go away. And it kept getting worse despite very little running and, finally, no running at all. So he was forced to give up the spring triathlons and focus only on bike racing. From that point on it has been amazing to see what has happened. His functional threshold power (FTP) has risen more than 20 watts. It's never been this high, or even close to it, in the last 4 years. He now finds himself near the front of his training group which includes several much younger riders instead of riding mid-pack and struggling to stay on during climbs. His bike sessions are remarkably different. The power numbers are 10 to 20% greater than for similar workouts done in previous years.
Even though the Achilles now appears ready to go again he's decided to stay with bike racing only for the remainder of the season. He's having too much fun to go back, he says. He refers to it as his new-found youth - at age 56.
Specificity is a powerful principle. Ralph's experience is a good lesson in this for all of us. If you really want to be as good at something as you can possibly be you need to keep this principle in mind. That doesn't mean you should never do anything else. Aerobic cross training and seriously lifting weights in the Prep and early Base periods is still beneficial, I believe. But the closer you get to your most important races the more like the races your training must become.