The first kilometres do not decide everything
The opening kilometres are rarely the place to gain time. Controlled pacing can reduce the risk of accumulating fatigue too early.
Average pace is only a reference
Terrain, weather, aid stations and how you feel can change instantaneous pace. The main goal is to manage effort so you can continue moving consistently.
The finish rewards management
Keeping something in reserve early gives you more options after 30 km. A late acceleration is optional; a major slowdown after starting too fast is much less so.
This information is general. Training should be adapted to your experience, recovery and personal situation.