When starting a workout, one finds that their muscles are stiff and painful. But with time, the muscles become more pliable and thus the pain leaves. As the person builds on their flexibility, they find that they can do much more than they started with. If it is weightlifting, the person realizes that they can lift more weight and that without much struggle.
Mountain climbers do not start with the highest mountain. In fact, they may start with smaller hills in their vicinity until they come to the mountains. What they are doing is building up their strength to tackle harder courses. In life, it is the same way. The small problems in our lives help us to build strength so that when we face difficult circumstances we do not give up. We learn to fight in the shadows so that when we get into the arena, we have the stamina to face off with other opponents.
Toughness comes from sparring. You may not win all the matches you get into, but they do not break you. You learn all the moves that an opponent can use against you and you know how to graciously step aside so that the punches do not land on you. Yes, not everything that comes against you will be good but you will have known how to navigate around it and come out better than you were.