The term pastoral letters (1 & 2 Timothy and Titus) is a bit of a misnomer. Timothy and Titus were not pastors in the sense in which the N.T. uses the word—that is, they did not limit their work to a single congregation as part of a group of mature men leading the congregation (see all of this in 1 Timothy 3:1-7; Titus 1:5-9; Acts 14:23). They were evangelists (2 Timothy 4:5).
An evangelist runs more along the lines of our understanding of a missionary. Their work, at least in the case of Timothy and Titus, is in establishing new congregations and setting them in order (see Titus 1:5). Furthermore, Timothy and Titus served as representatives of Paul. They were not apostles, but they worked for one! All of this means that when we read Timothy and Titus, it’s not an apostle addressing a church as we’re used to, it’s an apostle addressing his envoys in regard to the church.
The letters to Timothy and Titus then, represent a slight shift in terrain—from the flat, familiar ground of church letters to something just a little different—like going from an asphalt road to a concrete one—it will just sound and ride a little different.