A workout partner is someone who will be there for you. It is a person who has your same desire to succeed. A workout partner is someone who can't wait to train with you. It is someone who can't wait to be energized by your energy level.
Paul was writing to the church in Rome. He couldn't wait to see them and spend time with them. Paul was excited to help them train. He was excited to tell them about Jesus. Paul was excited to train with them and be mutually encouraged by them.