When you are ready to set your mission live and have your participants start adding distance you need to "launch" the mission.
Steps:
You can launch the mission from the mission page. Near the top there will be a message with a link that you can click to launch the mission. This will take you to the billing area
If you have enough funds in your wallet, the mission will automatically launch. If not, it will show you how much you need to pay for the amount of participants who have joined so far (or if 25 of less for the minimum charge).
Click on "Add Wallet Funds" to add the needed funds into your wallet to launch. More on funding your wallet here.
Once the funds have been added your mission will launch! People can then start posting distances and your time frame will begin counting down