Pick something you can see from here. Get fit is a mood. Run 5 km without stopping is a goal, because on the last day you'll know whether it happened.
Yours, and a bit too small
Make it yours. A goal you picked because everyone at work is doing it is a goal you'll drop in week three.
Make it a bit too small. If it feels slightly embarrassing, you've probably got it about right. You can always add a second goal next time.
Write down why
Then write down why. Not for us. For you, in week five, when the novelty has gone and it's raining.