A study done by researchers at the Joint Replacement Institute in Los Angeles, California, used the following guidelines to calculate activity level. This may help you:
Inactive 0 to 5,000 steps/day
Low activity 5,001 to 10,000 steps/day
Moderate activity 10,001 to 15,000 steps/day
High activity 15,001 to 20,000 steps/day
Extreme activity 20,001+ steps/day
You don't really have to measure every single day for the rest of your life. Their study showed that a four-day sample is long enough to give you an idea of your overall activity level.
You may want to recheck your results three or four times a year. This will account for seasonal changes. Repeating the study over the years will also give you some long-term calculations. This is especially helpful if you change your activity level over time.