Okay, I think I got an answer. "This generation" saw the "these things" that were to happen before the end, but not necessarily the end when Jesus is to come in the clouds and everyone is judged and the nations mourn. I found this helpful in my understanding as I was going through Mark with the New International Biblical Commentary, pp 222,223:
It is important to note carefully the wording of verses 29-30, or one might easily misinterpret Jesus' saying. The these things of verse 29 clearly refers to the "these things" of verse 4 and verse 8, and the "everything" (Greek, "all things") of verse 23, that is, the troubles that his disciples are to expect, including the destruction of Jerusalem. Logically, if verses 24-27 describe the end, then these verses cannot be the "these things" that assure one that an end will come! Thus, the these things of verse 30 must also refer, not to the end itself, but to the tribulations described in verses 5-23. These things will happen in the lifetime of the Twelve (v. 30), but that does not mean that the end will happen in so short a period of time.
Also, in verse 29, we are told that the events of verses 5-23 signal that the time is near, right at the door. The Greek reads literally, "it [or he] is near, at the doors," and what is referred to is probably the consummation described in verses 24-27. That is the distresses of verses 5-23 are to be taken as evidence that the consummation is surely coming. But this does not mean that the troubles are to be taken as indications that the end will happen immediately, for the whole point of verses 5-23 has been to debunk any attempt to calculate the date of the end by the occurrence of these events. . . ."