If I am doing a run but it takes so long I have to break inbetween am I allowed to pause my timer and restart it the next time I continue the run?

Editing videos together isn't allowed. The only category that allows for something similar is Perfection, which allows pausing and multiple video recordings (they don't need to be edited together) over a certain length of time (since most runs will require over 30 hours to complete in-game time).

All other runs don't allow you to pause (well you can take a break but your timer doesn't stop). All other runs require a single video where the run is completed in 1 sitting.

So a run like Community Center (CC) is anywhere from 2.5-6 hours (up to 10ish if you don't use a route) is to be done in one sitting with no pausing of the timer (can still take breaks but recording continues and you lose time to do so).