Pacing a crowded marathon start
Handle a slower opening kilometre without trying to win it back all at once.
Pre-write a decision
If the opening kilometre is congested, run with the available space and reassess once it opens. Weaving adds effort and creates sudden changes for runners behind you.
Use a rolling check
Compare elapsed time at a reliable marker with your pace band. Look at several kilometres and the course profile before deciding whether anything needs to change.
Example
A 4:00 target averages about 5:41/km. A 6:00 first kilometre is roughly 19 seconds behind that average. Chasing all 19 seconds in kilometre two would make it about 5:22. Your fallback can simply be to settle back into a sustainable effort.