It shouldn’t be that hard.
Of course at first you’ll be challenged because you’re switching from WordPress, but you’ll be OK.
I understand the demo site is a great learning tool, but starting from there in your case might not be the best course of action, as it will force you to do a lot of edits and changes to achieve what you want. The way I’d go about it if I were you (taking into account that you’re new to MODx, I would have started from scratch) is have a demo site handy for reference and make a clean install where you’ll start building your website.
Keep in mind MODx is pretty good at separating
content and
presentation. It means you can work with the demo site content as an example and take your WordPress theme and quickly turn it into a MODx one.
If I were you,
I’d start with presentation : this will be fast and gratifying
Have your WordPress theme handy. Strip it of all WordPress tags. Then copy/paste the html into MODx templates. And copy the CSS files into assets/templates/your-template/ (adjust path for your css in the template accordingly). You’ll quickly get the desired look.
Then and only then, worry about content. Have a look at the tags in your original WordPress theme : where are they placed ? what’s their function ? For each element, compare to the MODx demo site and start to dig into the logic. You’ll quickly manage to understand what is what, and insert the MODx tags into your MODx template. At the end of the process, you’ll have the same website that you had to begin with, plus of course MODx goodness
Now the last step is, sugar coating 
This is where you’ll add what’s not in WordPress but only in MODx. Look again at the demo site : what’s in there that you’re not familiar with ? Chunks, as in re-usable bit of contents. Plugins : they are different from WordPress plugins, essentially get familiar with System Events hooks and how you can enjoy it. Modules : hmmm, now you’ll see how you can add features to your manager

Last but not least, Template Variables ! Here is the mind boggling part... illimited custom content fields of any type which you can assign to templates. On top of that, bind them to data sources (be it database, file or whatever...), but that’s for much later when you’re at ease with the rest.
And of course : you have the forums along the process, to help you figure out the hard parts