This quote may be meant as comedy but it is completely valid, and something I say to clients all the time. I remember when I first tried salsa dancing. I went out on the floor, tried to do a few steps, and quickly said it wasn’t for me. In my mind at the time, I was right. It was crazy to think I could be a Latin dancer, and I convinced myself of this rather easily. Later on I took a few lessons, went out to practice many times, and eventually became (I like to think) an above average dancer, and created a very enjoyable hobby for myself for many years.
The same concept applies to so many other things. A relationship may be wrong for you or may be right, but going into it halfway with one foot out the door pretty much ensures it will not work. Trying any fitness program without giving it your full effort also will likely ensure a lack of success. The truth is that without jumping fully into something, you aren’t really giving it a fair chance. No one is saying you have to give anything a fair chance, that is always up to you, but you definitely cannot speak about its effectiveness or its applicability to you without fully going for it.