Every single part of your body, at every level shows some adaptation to training. When people first start running long distances, they often experience chafing, redness and other skin irritation in sensitive areas. Vendors are quick to grab on to that and supply us with all kinds of ointments, bandaids, nipple covers, and what have you. Inexperienced people are often alarmed by such phenomena and go out of their way to buy solutions. But the simple truth is that most-if not all- of these problems will go away with more training. There are some exceptions but they are few and far between.
When I first started running, my nipples were often sore. One time they started bleeding creating two red spots on my top and lots of weird stares from the crowd. I was unaware of the condition and wondering why people were looking at me in horror. Friends immediately started recommending various cures such as nipguards and other remedies, but after running several more races, the problem took care of itself. The same applies to nearly every problem one experiences. Just like muscle, your skin and your sweat glands have to adapt to increased exercise loads.
One more example, on my very first run -I was over 30 at the time- my head hurt so bad I had to stop running after less than 2 miles. That too went away after a few weeks without any special measures.
Unfortunately, not only are humans very susceptible to snake oil salesmen, they are also very prone to making spurious correlations. It is not uncommon to have an upset stomach when you first start running, or when you do your first long race. It is very tempting to correlate that upset stomach with something you ate or drank beforehand. But the most likely cause of such upset is the higher intensity of exercise that causes a relative hypoxia (low oxygen level) in the gut.
When I first started running I could not eat, nor could I conceive of anyone eating while running. After a few ironman races however, I was able to eat almost anything and keep running. It is not that I recommend eating while running. It does affect performance after all. It is just that one needs to realize that adaptation is universal and that a body can get used to nearly anything we throw at it.
Despite its global scope, training is very specific. You adapt to what you train for in a very specific manner. If you train to run, you will be able to run. Your cycling will also improve a bit but not nearly as much as you might think. To improve your cycling, you need to cycle.
The specificity is even more detailed than that. If you train by running long distances slowly, you will be prepared to run long and slow. You won't be fast and you won't get much faster this way either, no matter how long you train for. To get faster you need to train by running faster.
Yesterday I ran 11 miles in the hills. I did the full Shepherd Canyon loop, including the dirt hill, and then some extras at the end.