Usually the only reason for this is that they have prizes for both, and they don't want someone to win the "walk" entry when they've run a part of it. Walkers are generally not permitted to run. Runners, however, are permitted to walk (it doesn't give them an advantage).
So yes, you would sign up for "run" if you intend to run/walk.
It's absolutely normal to walk some!
Deb, in New Zealand