Find all videos less than x seconds in length

Check out the question at How can I search for videos with specific length or resolution? to add a length column and sort by it or use the advanced windows search features (it seems a little tricky trying to specify the max length).

You can enter this in the windows search box while in F:\video

kind:=video length:1..150,000,000

If you weren't sure of the specific length (specified in 100ns units) you could start with just entering the kind:=video and then it would search all your subfolders and you could apply the trick where you just sort on the length column.

Then just select the files from windows explorer, shift+right-click on them and select "Copy as Path" and you can paste them into notepad or a text file.