Obedience to God's word is not just some obligation to be fulfilled; it is the Christian's all. The King James version of this same verse says that it is the whole of man (disregarding the supplied word duty after whole). Furthermore, Deuteronomy 32:49 says this about keeping God's law; "For it is not a futile thing for you, because it is your life (...)" If ever I get so caught up in my earthly obligations that worship, study, and prayer begin to feel...well, just like any other obligations, I remember these verses and pray for God to restore my focus and zeal. He requires and deserves my all, and the effort is far from futile!
"(...) Be faithful until death, and I will give you the crown of life." Revelation 2:10