After the Jerusalem council there were things Paul and Barnabas had to straighten out in Antioch which took some time. The Jerusalem prophets, Judas and Silas, were sent along with Paul and Barnabas to assure the gentiles that the message Paul brought back from Jerusalem was indeed the decision of the apostles there (Acts 15:22). In this way misunderstanding and any possible mistrust was rendered moot.
From the time of Paul’s first missionary journey to the point of his second must have been about two years. Paul had to have spent several months with the churches from Cyprus in the Mediterranean to Derbe in Galatia and then back again, retracing their steps and confirming the churches as they went. When he finally returned to Antioch in Syria he was there awhile before “those from James” arrived. triggering the debate which ended in the Jerusalem council convened in Acts 15. If Paul and Barnabas began their first missionary journey in spring or early summer he would not have returned to Antioch before fall, but certainly by winter, because sailing during the winter months was hazardous and would have been avoided. If the foregoing is reasonably accurate, those from James, may not have arrived in Antioch until winter or early spring the following year. The Jerusalem council would not have convened until sometime in the summer, and Paul and Barnabas would not have returned to Antioch for a few weeks later, possibly mid to late summer, depending upon when the council took place.
During this time, Paul heard about the churches of Galatia being troubled by those holding to the doctrine of circumcising the gentiles. It may be that the circumcision party at Jerusalem sent several groups to where Paul’s ministry took him, and Antioch was only one of those groups. Paul wrote the epistle to the Galatians, while he was in Antioch, but couldn’t come to them, due to the fact he had the work of confirming the gentile churches of Antioch, the rest of Syria and those in Cilicia to whom the apostles in Jerusalem sent letters concerning the circumcision controversy (Acts 15:23), implying more than one group sent out to the gentile churches from the “circumcision party” at Jerusalem. Therefore, Paul wouldn’t have been able to sail to them before the spring of the following year or two years, since the time he and Barnabas had raised up churches there.
I mentioned “sailing” above, because this was the probable plan before Barnabas and Paul split. The two may have planned to encourage the brethren on Cyprus (which is where Barnabas and Mark sailed according to Acts 15:39), then visiting the churches on the mainland and perhaps going to other nearby towns or perhaps even the north African coast. Nevertheless, since the two split, Paul and Silas traveled by land. Paul visited Derbe, Lystra, Iconium and Antioch in Pisidia (Acts 16:1-6), but there is no mention of his visiting Perga, one of the cities of his first journey. Barnabas and Mark may have visited Perga after going to Cyprus and then to perhaps additional cities in the plan. It appears that Paul and company had no additional plans after visiting Antioch in Pisidia, which seems odd, since he didn’t have any intention to return to Antioch in Syria, after confirming the churches visited on his first missionary journey. Therefore, I suspect Barnabas and Mark followed the original plan, minus the Galatian cities and Antioch in Pisidia.
Notice, that Paul took with him the letters from the apostles (Acts 16:4) to prove what he would say to them, as it pertained to the doctrine of circumcision and new gentile Christians. Silas, one of the original leading men of Jerusalem sent with the letters to Antioch, Syria and Cilicia, travelled with Paul to Galatia and was able to confirm his words and thus render any contrary argument moot. Since Mark went with Paul and Barnabas on their original missionary journey as far as Perga, he would have been in position to be the confirming witness to anything Barnabas had said concerning the troubling doctrine, especially if Mark was originally against the gentiles becoming believers without first becoming Jews.
It must be emphasized that this is only my thoughts, and while they seem to fit the context, they are by no means set in cement. I offer them only to help explain some questions I see in the text. May our great Lord and Savior guard his word against misuse.