We didn't design World Walking with time-limited walking challenges in mind but groups do use it for this.
If you have banked steps prior to your challenge starting then I suggest you start another walk just for yourself (solo) and then spend the steps you have banked before your challenge starts on that solo walk. That way you can start your challenge with a zero balance in your bank of steps.
Once your challenge has ended you can go back and complete your solo walk if you like.