It is possible to both have your family spectate during the race and greet you in the family reunion area afterwards and enjoy the parks with them before and/or after the race. Just remember to clearly define expectations before the trip and make sure that everyone understands that. You may have to take it easier in the parks than them. Maybe this involves sitting on a bench and resting while they stand in line for a ride. Perhaps it involves doing more shows than normal in order to minimize time on your feet before the race. It may well involve you going to bed early the night before the race while they stay out and play. My mom came down to watch me run a couple of races in
Disneyland. She understood that I would be going to bed very early the night before the races and needed a quiet dark room in order to get the rest I wanted before a race. Having run many races by myself and thus relying on either self motivation or the kindness of spectators cheering for everyone, it is really nice to have family there to celebrate with. My mom spectated during the 2015 Star Wars 10K and Half at Disneyland. My little sister spectated during the 2017 Star Wars 10K at Disneyland. It is really fun to see family members there knowing they're cheering especially for you. If this is a special race for you going in, do not be afraid to miss out on the experience of sharing that with your family. Just set reasonable expectations beforehand.
I would also recommend clearly defining expectations with your friend. I have run 5 races now with my little sister. Or more accurately, we have ran in the same race 5 times. We have never run together because we run at different speeds and do not want to impact the experience the other runner wants to have. If we had run together, both of us probably would have had very different experiences. I stopped for a whole lot of pictures during the all 3 races. My sister stopped for 2. In the end, we both enjoyed the races equally, but on our own terms. While it would have been fun to have a picture with me and her with the Ewoks since I gave her the first plush Ewok that became her very favorite plush toy in the entire history of toys, we both had better experiences doing our own race as it were. And we did get a photo together after the race with Darth Vader.
If you and your friend are on the same page, then great. Enjoy it. Just make sure going in that you both know what each other wants out of the race. You don't want one person to have dreams of setting a personal record and the other one hoping to stop for as many character photos as possible running together if those 2 goals are very incompatible.