Jayme Edwards I had to learn the hard way. haha. It took me too long to realise all this. I'm a co-founder too so I might be a bit more of an outlier but I feel it all still applies, possibly even to most things in life.
That said, one shouldn't feel guilty if one needs to get something done outside of the "allotted time" if need be. I still have lots of ideas about work when I'm outside of it and write it down, or do a spot of research here and there, or if there's an email/discussion that's important then deal with it, only if it's important though.