If you are legally married but have not lived together for over 10 years. Can you file as a single (unmarried for IRS)?

No.   You must have a divorce decree or a legal separation decree with spousal maintenance to file single.   (legal separation is only available in a few states and is almost the same as divorce so is rarely used.)

No, you cannot use filing status Singe, however If you are legally married, you can still be considered unmarried in the eyes of the IRS if you didn't live with your spouse for the last half of the year. You can file as head of household, if you have qualified  dependent and other requirements are met. 

Divorced or Separated Individuals

no you can't file as single.  until divorce, entry of separate maintenance agreement, or death of spouse, the IRS considers you married. so the choice is either a joint return or married filing separate return