Overlapping in TOC when using default section prefix with renewcommand thesection

I’m writing on an exercise report and I want to have the prefix "Exercise" in my section headings (eg. Exercise 1: The QGIS user interface). So I used renewcommand*{thesection}{Exercise~arabic{section}:}. But now in my TOC the spacing is not correct. The prefix and the heading are overlapping:

documentclass{scrreprt}
renewcommand*{thesection}{Exercise~arabic{section}:}
begin{document}
tableofcontents
section{The QGIS user Interface}
Text, text, text...
end{document}

How can I fix this?