Start small. Do more or add more if you want to.
This applies on many levels.
For example, if you’re trying to lose weight, take less than you want. (You can always go back from more but can’t give back what you eat.)
If you’re starting an exercise program, commit to a smaller realistic amount rather than an unwieldy longer time. (You can always add more.)
If you’re cleaning your house, promise yourself you’ll do one room really well instead of the whole house and get overwhelmed. (You can always do more if you want.)