The running headers are occasionally too long, I tried using \sectionmark but it doesn't work...
MWE:
\documentclass[twocolumn,a4paper,10pt]{book}
\usepackage{fancyhdr}
\pagestyle{fancy}
\usepackage{lipsum}
\begin{document}
\tableofcontents
\section{Introduction}
\lipsum{5}
\section{Here's long text that is two long for the page and I'd like to substitute it but can't figure out how}
\sectionmark[long text]
\lipsum{5}
\end{document}