I love this quote from Brigham Young as I can certainly relate to it from the perspective of my own testimony.  While reading the scriptures and prayer have certainly helped me build a foundation, I think the overwhelming majority of my testimony has been gleaned on my feet.  I have had wonderful experiences in service, particularly working with youth in the Church.  Without these experiences, I feel my testimony would go stale regardless of how much praying and scripture reading I do.   From my limited experience, it seems that in the Church, there are two types of people with testimonies – knee people and feet people.  Knee people tend to pray and study the scriptures and gain tremendous testimonies in doing so (they also serve, but their testimony is primarily built on their knees and not on their feet).  Feet people tend to have their testimonies built on a foundation of the scripture/prayer, but the bulk of their testimony is built from the service they perform.  While neither is better than the other, both are critical for the success of the Church.  Which are you?  Why?