The tricky part about DITHOR is not whether you're well versed in Lit studies, but just getting used to the program as a whole. Not every Day of DITHOR has a page in the workbooks - sometimes you need to just follow instructions from the TM; some days you need both. Also, since you don't do DITHOR every day, the DITHOR Days (15 reading days and 5 project days) don't correspond with any days from the guide. Once you've gone through a few genres it's much easier.
However, if you look at numbers of scheduled DITHOR days, for Preparing and up you do DITHOR 3x/Unit, so 3x35 units is 105 days. If you get 5 genres done, that is only 100 days (20 days/genre, as mentioned above) so that really gives you an extra 5 days throughout the year (according to the guide) for days off, or a separate day as an intro to each genre, etc. However, if it's your first year with HOD, I recommend planning to do only 4 genres (of course, doing 5 in the second year means you'll cover all 9 and aren't "behind" at all!
) If you do just 4 genres, then there are actually 25 "extra" DITHOR days scheduled in the guide so you don't have to start until Unit 9. . . or start at Unit 5 and take a Unit off of DITHOR between genres (and one at the end), or. . . whatever works for you!
As for difficulty, because they are stretching their reading, vocabulary, etc. through the other independent reading (at least from Preparing on up), the goal of DITHOR is not to stretch their grade level. You want DITHOR books to be slightly easier than the average "school" book, so they can spend their efforts, not decoding, but analyzing the literature for the elements (plot, characters, mood, setting, etc.) and the Christian character highlighted in each genre. So, my suggestion is, if she's reading at a grade 8 level, maybe start with the 5/6, or at the highest the 6/7 book pack for DITHOR. If you do just 4 books/genres this year, you can continue on with the rest of the book pack next year, or if you find they aren't challenging enough, you could give them to her as "free reads" on her own time over the summer (since, even if they're too "easy" for her for school, they're all wonderful reads!) and move up a level (or two, based on your observations and her abilities) for next year.
I hope that helps a bit.
Blessings,