Building a nav from the top level page of a structure
Untested, but you should be able to do this by using the merge filter in Twig to join the two results together.

Get the top level 'landing page' for this section:

{% set landingPage = entry.getAncestors().level(1) %}


Get the descendants for this section:

{% set childPages = entry.getAncestors().level(1).getDescendants() %}


Merge the two results together:

{% set sectionPages = [ landingPage ]|merge(childPages) %}


Now create the navigation for the section using the sectionPages variable:

&lt;nav&gt;
    &lt;ul&gt;
        {% nav page in sectionPages %}
            &lt;li&gt;
                {{ page.getLink() }}

                {% ifchildren %}
                    &lt;ul&gt;
                        {% children %}
                    &lt;/ul&gt;
                {% endifchildren %}
            &lt;/li&gt;
        {% endnav %}
    &lt;/ul&gt;
&lt;/nav&gt;