Their first assistant was a coward. We all know about Peter and his denial of Christ, but he didnt become the pristine example of purity after that either. In fact, in the Bible he is referred to as a hypocrite. Peter, Paul, and Barnabas are our heroes too.
Have never heard of this John Mark being called a coward.
IIRC, correctly, John Mark abandoned his first missionary journey.
Barnabas then took him under his wing.
IMO, the real heroes that many over look are Stephen, Barnabas, Tychicus, etc. The ones who Paul commends who served.