There's a lot in common with the 10K (my review here: http://bit.ly/1V4Zf6V), but the biggest difference is where the course had to spread out to make 21km. In this case, the area directly south of Disneyland. Inside the parks it's all magic, then you run past nail salons, boarded up houses, McMansions and law firms of varying levels of distinction. I didn't mind this too much, but it's a lot less magical when you leave the parks and you're surrounded by the Real World. They did their best to keep the Star Wars theme throughout, mostly through recruiting cosplayers (like the 501st Stormtrooper Legion) to encourage near the aid stations. That all said, the course was well managed and they did their best. Also,nothing quite compares to running through he parks and backstage before sunrise, or that finish line!

In retrospect you're basically running throughout the Disneyland Park and California Adventure Park with plenty of backstage sections (apart from the aforementioned LA jaunt). During this run, they pipe in Star Wars theme music along the entire course, with random Star Wars Character photo-ops and mile marker photo-ops, as well as random Star Wars touches to keep an eye out for. This, as a prequel-hardened Star Wars fan, pleases me.
The downside is that there will be a long queue for each photo-op, which I just couldn't make myself do (not with all those runners passing me en-masse). The second bummer is the newly-tightened costume restrictions, effectively killing my custom Jedi cosplay glory. I'm sure they had their reasons, but there shouldn't be an appreciable difference between a cape and a cloak for safety or security reasons. Sleeve prejudice I say!
But I digress...
Great expo, worth arriving the day before the event. Unlike what some have said, they don't run out of merch! A queue at the expo means good things: have patience.

I did this event as part of the Star Wars Rebel Challenge, which is this 10K followed by the Half the following day. My only complaint about this is I wish there was a another level of challenge which included the 5K the day before (a la. Goofy Challenge at WDW).

Lastly, the crowds. Apparently the first they did this Star Wars Marathon weekend in 2015, the crowds weren't too bad. This year, what with the confluence of the new Star Wars movie a month earlier, Star Wars Season of the Force event in Tomorrowland, and the Official Diamond Celebration filming over a 3-day weekend, this place was a mad house! My family and I took refuge in California Adventure for most of the time here, as there was literally nowhere to stand, let alone walk or play. It might not be that bad in 2017, but bear this in mind when signing up.