You have 2 options, the hard way ~ the route you have chosen ;) and the easy way... which I will outline as well.
For what you are trying to do you are going to want to use either getResources http://rtfm.modx.com/display/ADDON/getResources which will allow you to loop over a collection of resources & extract the fields you need [in this case 'content'] or getresourceField http://rtfm.modx.com/display/ADDON/getResourceField which will allow you to specify a single resource & the field to extract, just call it multiple times in your template.
You might try:
[[getResources? &parents='[[*id]]' &tpl='myTpl' &includeContent='1']]
Should get all the child resources [up to default ~ 10] of the current resource, then you would create a template for get resources to loop over:
<h1>[[+longtitle]]</h1>
<p>[[+content]]</p>
notice the use of + instead of * for the resource fields
OR
the easy way, create template variables for your extra content areas, you can set them up as rich text areas as well. that way all your content for any given page is within the one resource ~ no need for you to create child resources to hold content for your header/footer/sidebar/etc
Either way will work, the TV method may use less overhead though.