From Ramsey’s* God, Christ, and the World (pp. 48-49):
Demythologizing was taking place in the apostolic age. In the teaching of Jesus there were pictures of a future coming of the Son of Man on the clouds and of the establishment of a divine kingdom described in vivid apocalyptic imagery with the details of a final judgment. In some of the sayings of Jesus these things were to happen within the lifetime of the disciples. But was it possible to expect things to happen on the scene of history just like that? Or were there underlying realities which the imagery conveyed to people in a certain setting of thought and culture in Palestine and which other imagery would have to convey to people in another setting of thought and culture? The teaching of the Fourth Gospel about the return of Christ through the indwelling of the Holy Spirit and about the realization of eternal life and divine judgment by the Christian in the here and now, may fairly be called a ‘demythologizing’ of the earlier imagery. Again, the spatial imagery of a local heaven to which Jesus was exalted at the Ascension was seen to convey realities altogether beyond space–the sovereignty and omnipresence of Jesus. It would be quite untrue to say that a single mythological frame dominated the thinking and teaching of the apostolic age. The records contain varieties of myth and varieties of demythologizing at work. Factual records, myths, demythologizing propositions and sometimes–as in the Apocalypse–‘remythologizing’ processes all had their part in the apostolic thinking, teaching and writing about Jesus Christ.
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*Ramsey was the 100th Archbishop of Canterbury, serving from 1961 to 1974. See here for more.
A Thinking Reed 
“In some of the sayings of Jesus these things were to happen within the lifetime of the disciples.”
There are ways of reading where you don’t run into that. For instance “Truly, I say to you, this generation will not pass away until all these things take place” reads one way if you read “generation” as a unit of time, and another if you don’t. From what I’ve seen, this is a word with no easy English equivalent. I would start with something like “descendant[s] of an ancestor.”
Before I went to demythologizing, I’d want to check whether there was another way of reading the passage that didn’t involve all the assumptions that led to the problem.
That’s a fair point. I think Ramey’s point that you have diverse ways of conceptualizing this in the NT still stands, though (the apocalypticism of the synoptics vs. the “realized” eschatology of John, e.g.).