I'd say the hardest bit about running a business is not running the business. It's everything else - admin, figuring out what you actually want to do, reaching customers, retaining customers etc.

But also that's the bit you have lots of time to figure out. Focus on the bits you enjoy and try to learn as much as you can (unless you are really desperate for cash out of this, in which case focus on all that!). You can figure out the boring bits later.