You cannot leave a team while the challenge is in progress (when you are able to start adding distance), but may do so before or after.
To leave a team, simply log into your challenge, click on 'My Team' in the navigation, click on the cog icon in the top right hand corner of your team's box and select "Leave Team".
Team admins cannot leave a team without first appointing a new admin to take their place. You will need to do this by contacting us directly.