Why is it important to determine your fitness level before designing a fitness plan?

Your general health. if you are perfectly healthy then you can start one. If you know your base fitness, then you know how to not overdo your plan. Once you are walking at least 15 to 20 minutes at a brisk pace daily, then you can take the One-Mile-Test. It will give you an estimate of the endurance fitness you have and then you can improve and measure from time to time. Look it up.