Martin Luther King, Jr. was a leading force in the civil rights movement in the late 50's and 60's. Dr. King worked hard to create peaceful protests against the injustices that were going on in the deep south against African Americans. 

The statement "if you can't fly then run, if you can't run then walk, if you can't walk then crawl, but whatever you do you have to keep moving forward" was said at a rally at a college. What Dr. King was saying, is that no matter what you can do, you have to keep pushing forward in the fight for justice. He was trying to get students involved in peaceful demonstrations he was doing. He knew that young people were going to be the leaders one day, and they had to get involved. Dr. King was a preacher and he used his preaching as way of life. He used the Gospels as a way of modeling his fight for justice. This statement is actually taken from Isaiah 40:31. He wanted people to see that you could make a difference and still be a peaceful human being. Yes, he was a fighter for the rights of people, but his life as a preacher is what led him.

"Before I was a civil rights leader, I was a preacher of the Gospel. This was my first calling and it still remains my greatest commitment. You know, actually all that I do in civil rights I do because I consider it a part of my ministry. I have no other ambition in life but to achieve excellence in the Christian ministry. I don't plan to run for any political office. I don't plan to do anything but remain a preacher. And what I'm doing in this struggle, along with many others, grows out of my feeling that the preacher must be concerned about the whole man."

Dr. King made this statement in 1967 and this sums up who he was. He cared about all people. He was just as concerned about the poor people, as he was the African American community. He was a peaceful man and that was the way he lead his civil rights work. He looked at as being called by God. After all these years, his vision still lives on today. If more people were willing to act and live their lives as he did, wouldn't this world be such a better place?

Martin Luther King, Jr. is best known as the most important leader of the Civil Rights Movement in which African Americans did whatever they could to fight for justice.  They did this even when it seemed like they would never win.  This sort of determination is what King is talking about in this quote.  He is saying that no matter what your limitations are, you have to do as much as you can to improve.  You have to forget about what you can't do and instead just focus on doing as much as possible.  If you do this, you can end up accomplishing things even when they seem impossible.