Why was Peter the most likely source for Mark's Gospel?

A:

The second-century Church Father seems to have thought that the Gospel was actually written by Mark and that he wrote down what Peter told him about the mission of Jesus. Apart from that, it seems very unlikely that Peter was at all involved in writing Mark's Gospel, which was originally anonymous and can not with any certainty be attributed to Mark.

Traditionally, Peter is said to have died in Rome around 64-66 CE. Even if this is not really true, it seems unlikely that, as a disciple of Jesus in 29 CE, he was still alive in 70 CE when this Gospel was written. On the one hand, why would he wait for decades after the crucifixion of Jesus before passing on his story; on the other hand, why would the author of Mark wait so long after the death of Peter before writing down what Peter might have told him? Furthermore, the author of Mark's Gospel was quite familiar with Greek mythology, whereas Peter appears not likely to have had the same education.