O modelo que você está procurando é "content.php"
Você desejará alterar esta linha:
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
<?php the_excerpt(); ?>
</div><!-- .entry-summary -->
<?php else : ?>
<div class="entry-content">
<?php the_content( __( 'Continue reading <span class="meta-nav">→</span>', 'twentyeleven' ) ); ?>
<?php wp_link_pages( array( 'before' => '<div class="page-link"><span>' . __( 'Pages:', 'twentyeleven' ) . '</span>', 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php endif; ?>
Para isso:
<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
<?php the_excerpt(); ?>
</div><!-- .entry-summary -->
<?php else : ?>
<div class="entry-content">
<?php the_excerpt(); ?>
<?php wp_link_pages( array( 'before' => '<div class="page-link"><span>' . __( 'Pages:', 'twentyeleven' ) . '</span>', 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php endif; ?>
Observe a mudança de the_content()
para the_excerpt()