Inherited IRA Beneficiary - No RMDs taken

Hello Tax Experts!

My dad passed in 2012 (age 84), but I didn't receive his inherited IRA (traditional) until July 2015. I thought RMDs were all based on the living person's age (I'm not 65 or 70 yet!), so I just let the account sit as is.

My mutual fund company alerted my that RMDs need not be taken for 2019 (CARES Act), which prompted me to ask why I was alerted of this (thinking I hadn't hit the milestone years). 

 

Given my situation, what is the best method for fixing my errors, and if I need to withdraw, how do I know how much?

 

Thank you in advance!