Titus 1:5

For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and ordain elders in every city, as I had appointed thee:

Paul is not taking the place of church authority. The apostles were supposed to train future generations because they had this authority from Christ Himself. In this they were totally unlike the other ministers of the day who always got their authority from the local churches. The work of the apostles was to be over the churches. This office has been terminated because there is nobody who fulfills the qualifications today. See Acts 1 for a list of the qualifications. Verse 1 states the office Paul is speaking from, which is an apostle.