Testing etoolbox's AtEndPreamble command to reference author, title, date declared at bottom of a preamble.

AtEndPreambleMWE.tex

\documentclass[12pt,a4paper]{article}

\usepackage{lipsum}  

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Margin Start
\RequirePackage{fancyhdr}
\pagestyle{fancy}
% clears the header and footer
\fancyhf{} 
% Keeps header and footer same size text when adjusting margins
\fancyhfoffset{0pt} 

\usepackage{etoolbox} % For \AtEndPreamble

% Keep \makeatletter and \makeatother out of \AtEndPreamble{}
\makeatletter
\AtEndPreamble{%
  \let\runauthor\@author
  \let\runtitle\@title 
  \let\rundate\@date
  \lhead{\it\runtitle}
  \rhead{\it\runauthor}
  \lfoot{\it\rundate}
  \rfoot{\it\thepage}
}
\makeatother
% Margin End
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\author{Joe Blow}
\title{Great Work}
\date{2021-07-04}

\begin{document}
  
\maketitle
 
\lipsum[1-10]  

\end{document}