I'm sorry if this is a stupid question but before I run a level I usually start up a recording and leave it up until I get a new PR. Due to this I typically have big bulky 20-minute-long videos of which only 20 seconds are relevant. Am I allowed to cut all the bits before the run for convenience's sake, or do I just upload the minor behemoth that is my submissions and accept the fact that the poor moderator is probably going to have to scrub it clean to make sure I didn't cheat 12 minutes in.

You can cut out all the unnecessary bits, just make sure to put in a few seconds before the actual run.

Please do. Session recordings are not necessary unless we ask you for them specifically.