Hi - I run three successful companies of varying sizes. It can be done, but it is not easy to accomplish and it is easy to get sidetracked if you are not intentional about developing focus each and every day.

I've written and podcasted a fair bit about this subject (http://wisenapkin.com/tag/focus/) but the key is this: develop a very clear picture of where you're going; develop a top 5 list each and every day of the most important things that you can do that day to work toward your goal, maintain records of your progress, ask for help along the way, get a coach/mentor/forum of your peers, and execute consistently.

Make it a great day!

If you're one person, focus on one.

If you can build a team around each that run it day to day - then yes, but typically people who ask this question aren't in that position :)

Like most skills in life, focus is something we must develop and practice especially in this ADD world of ours where there's always some new shiny object trying to hook our attention. If you know it's a challenge for you - then the best way to start is to create an environment for yourself that is conducive to focus.
For example, creating a time-block calendar where you work in blocks of time where nothing is present to distract you from the task at hand. So if you want to write - start first thing in the morning and set one hour aside for that creative stream to flow BEFORE engaging your email or any other "left brain" type activities. Shut the phone, close the door, and just write whatever no matter what it is. Keep doing that at the designated time on a regular basis and you will train your brain to respond accordingly. Obviously apply this to whatever you want to focus on.
Using a time-block calendar is a great habit to put into practice. It cuts down on overwhelm, confusion, and all the other crap that prevents entrepreneurs from being as successful as they could be.