The Between Two Worlds blogsite has a very thoughtful post on duty and legalism by John Piper.
There is a tendency in the church today to dismiss duty as some sort of manifestation of legalism. The result is sometimes, perhaps often, that we ditch disciplined Bible reading and prayer as "legalistic". Unfortunately in our desire to avoid what we mistakenly think to be a legalistic approach to prayer and Bible reading, we don't get around to doing much of either. Devotional practice becomes spasmodic rather than habitual.
You can read what Piper has to say here