Do you understand we're in the last days?
No. And neither do you because you don't actually know this, it's nothing else than an opinion of yours. As such "understand" is the wrong word.
If you don’t believe that we are in the last days, why not?
Why would I? There is no reason to believe it is and Christ himself said we wouldn't know the hour or the day.
Finally I don't think it is all that important really. As far as your devotion and moral life is concerned it shouldn't and doesn't matter. It isn't more or less important to live a Christian life 1 day or 10 000 years away from Judgement Day.
Subjectively speaking we are constantly living in the last of days anyway. Because all of our days are numbered. Just like the actual last days we don't know when it will occur and you could get hit by a truck tomorrow. The sentiment of "make me holy, Lord, but not today" is a lie. You don't even know if you have any time beyond today to get holy on.