{# Get all of the entries entered by the user. #}
{% set entries = craft.entries({
    'section': 'mySection',
    'level': 2,
    'descendantOf': user.id
}) %}

{% if entries|length %}

    ...

    {% set value = 0 %}
    {% set count = 0 %}

    {% for entry in entires %}
        {% set value = value + entry.distance %}
        {% set count = loop.index %}
    {% endfor %}

    {% set averageDistance = (value / count)|round(1, 'floor') %}

    <p>Avg: {{ averageDistance }}</p>

    ...

{% else %}
    {# Could not find any entries. #}
{% endif %}