If I start withdrawing from my 401K after I reach 70 years old, will I have to pay Federal Income Tax?

Yes, withdrawals from a 401(k) are taxable, assuming it's a regular 401(k) and not a Roth 401(k). There's a minor exception if some of your account consists of after-tax conftributions, but that's pretty rare in a reguar 401(k) account.