I find the following mind-boggling.

Suppose that runner $R_1$ runs distance $[0,d_1]$ with average speed $v_1$. Runner $R_2$ runs $[0,d_2]$ with $d_2>d_1$ and with average speed $v_2 > v_1$. I would have thought that by some application of the intermediate value theorem we can find a subinterval $I\subseteq [0,d_2]$ having length $d_1$ such that $R_2$ had average speed at least $v_1$ on $I$. This is not necessarily so!

Question. What is the smallest value of $C\in\mathbb{R}$ with $C>1$ and the following property?

Whenever $d_2>d_1$, and $R_2$ runs $[0,d_2]$ with average speed $Cv_1$, then there is a subinterval $I\subseteq [0,d_2]$ having length $d_1$ such that $R_2$ had average speed at least $v_1$ on $I$.