The average comes from Strava and is based on moving speed.
Afaik, the only way this would be influenced by (long) breaks, is if you have recorded the activity on a device, or in a way, that doesn’t distinguish this.
I.e. on most bike computers (and I think also in most mobile apps), you can set auto-pause on, which would record both. Strava will then only use your moving time to calculate averages.