have you tried just writing all of it without bothering to pay attention to the time and then when you're done writing test record and then cutting/editing until you get it around your ideal time limit
when writing such a long video, it is actually important to record (even just a crappy version) of me speaking the script, so that i can hear how it's going. i actually do this when i'm writing anything. a side effect of this is you can see the time, but that's not the reason i do it