Start With Where the Arch Sits
The arch's position decides the design more than anything else. Work out which of these you are doing before you choose a layout.
A start or finish line is the classic use, and it is the one that gets photographed most. Every finisher photo has your arch in it, often with the timing clock and the runner underneath. That makes the top of the span the most valuable space you own.
An entrance arch marks a way in, so the priority is that people recognise it as the entrance from a distance and know whose event it is when they get there.
A sponsor arch exists to give a backer visibility in exchange for their money, which means the design brief is theirs as much as yours, and the space needs planning rather than improvising.