NEW SOLUTION- So, I deleted my old solution cause the video I posted had 170 views. It was called "Videoland 3-D". Anyway, here's a very very easy way to find long videos. Guranteed to work! Follow these steps....
1)Go on YouTube.com from a PC
2) Search, "Long videos" (Press enter)
3)Press "filters" below the search bar and you should see "Upload date" and "Duration"
4) Select "last hour"
5) Now, select "Long (20~minutes)
And... Voila! There it is. You should find tons of long videos. Comment or message me if you're having trouble.
EDIT- Hey, they got off their lazy money hungry asses and fixed the achievement!