Adopt a Sense of Urgency

In the startup world, there is never a dull moment. Wait, I take it back: there may be times with a boring task or project, but there are never times where there's nothing to do.
There are always new tasks, new action items, new goals, and new...you get the drift.
Of everything that comes up, some ideas are important right now and some are keepers, some tasks are critical for growth and some are nice-to-have, and some things are neither but need to be attended to as supporting tasks for everything else.
Oh, and there are also things you need to do that may not fall within your responsibility, but someone's gotta do them...and it may as well be...you :)
Whatever is within your daily, weekly, or overall task list, adopt a sense of urgency and get things done. No doubt about it.
Figure out what you want to achieve and attend to it by priority. Since you don't know what will actually hit, you need to get started fast, launch, and assess results so you're not wasting time and things keep moving...right now.
Without that sense of urgency, your startup will be checking off tasks while missing extraordinary opportunities you didn't even know existed. Trying things quickly, pushing in this direction or that, going deep here and there is part of the process.
Attending to things as urgent matters (unless they're completely not) will reveal a whole new world of ideas, possibilities, growth directions, and feedback to help you get better, bigger, and stronger...much faster.