Everyone has different ideas on strength. Maybe strength is lifting one hundred pound weights, going to work every day, or maybe even just letting go of your past.
Strength is not a set standard, and is not defined by the opinions of others. True strength, I believe, is doing what you know is right even when it's painful or hard. Strength is pushing yourself to do something you never thought you could do. It's loving your enemies, smiling when you need to cry, and pushing past human nature to be the best you can be.
I'm not going to tell you exactly what strength is. After all, for everyone it's different. But when the situation comes, strength is there. We just have to learn to be strong.