If you feel ready for the 10k, do that. If you feel unsure, do the 5k. You can always do the 10k later! I'm thinking it could be discouraging if you don't make it through, but on the other hand, if you're a competitive person it might also boost your determination to improve.
How much and how long are you able to walk? How often did you walk the last 4 weeks? Listen to yourself.. what does your mind say, when you think about walking for 10km?