Sometimes the rule is:
You don’t have to finish, but you do have to start.
And sometimes the rule is:
You don’t have to start, but if you do, you have to finish.
When building a personal habit, it might make sense to embrace the first rule. You don’t have to run all the way, every day, but you do have to get out of the house and start running.
And when making promises to a group where trust matters, the second rule definitely applies.