Well, first of all, I have not made that claim.
Jesus himself said that his return would be within the lives of 'this generation'.
Clearly, you are referring to Matthew 23:30. I am sure you are aware that there is a great deal of debate even among Christians with respect to this point - hence the preterist versus the pre-tribs, etc. I hold to the belief that it is a dual prophecy - the first being the destruction of the temple in 70AD (minor) and the second to his return (major). The word "generation" is used to describe those alive at the time with respect to the first prophecy but it also can be used to describe a people. In the latter case, I believe what Jesus is saying is that the Jews will not disappear from the earth before his return.
If we dig a little deeper into the passage that you quote (Matthew 23:30) we see that just prior to the "this generation" statement, versus 26 and 27 Jesus says
"At that time men will see the Son of Man coming in clouds with great power and glory. And he will send his angels and gather his elect from the four winds, from the ends of the earth to the ends of the heavens.
This portion is clearly not related to the destruction of the temple in 70AD. To sum this up, I believe that the prophecy is a dual prophecy describing the destruction of the temple in 70AD (a relatively minor event) as well as describing Jesus' return (the most important event).
It wasn't until 2 Peter that we see them beginning to try to come up with reasons for the delay of Jesus's expcted return.
Obviously, you are referring to 2 Peter 3:8. This response I found on the Net to preterist concerning 2 Peter is helpful.
It has been suggested (by preterists) that it is inconsistent or rather gratuitous to appeal to 2Pet.3:8 to explain the "delay" in Christ's return. "With the Lord one day is as a thousand years, and a thousand years as a day." But it must be noted that this is Peter's own explanation. The preterist argues that since Christ spoke of the "nearness" of His coming it must be that He has already come. The scoffers whom Peter addressed argued that the same evidence indicated that He will not come at all. And it is Peter's answer that leads us to think in terms of God's view of time. "For God, it's not been long at all!" This is an entirely legitimate argument. It has inspired apostolic endorsement.