Hi Andrew,
You can not take the result of an outbound calls and update a field with it. For your scenario, the following can work:
1. Perform an outbound call to a web service you develop on your side
2. Have the service call the Google Maps APIs
3. In your web service, take the result of the distance and call our web service API to update the relevant field.
Let me know if this can work for you.
Eyal