getting ready for the jump

okay, here goes.  I'm a newbie who is considering my first du.  5k, 20k,5k.  I know I can do the first two parts but what if I have to walk the last 5k?  I don't mind finishing last, I don't have any previous or current injuries to worry about, and I know that the people there will be so inspiring (shout out to michiga mafia!).  I guess I am just a little nervous...any thoughts...