Take a closer look at what Paul actually says in 1 Corinthians 15.
Last of all, as to one untimely born, he appeared also to me. (1 Corinthians 15:8)
I don't think you'd argue that Paul's experience on the road to Damascus was anything other than a vision. Yet, writing about it here, Paul uses the same word -- "appeared" -- to describe both his own personal experience and the experiences of Cephas, the twelve, etc. The implication I draw from this is that their experiences were of the same nature as Paul's. Or, at least, Paul thought they were. Paul says nothing here about the apostles touching Jesus, or him eating with them. In fact, later in the same chapter, he emphasizes the spiritual nature of Jesus's resurrection.
My point was to show that the resurrection appearances were known and discussed far earlier than the Gospel of Mark. The nature of Jesus' resurrected body is a different question, one that Christians themselves have debated down the centuries; which isn't surprising because resurrection is beyond our known science.