Choose an activity that you enjoy. If you do not like what you are doing, you probably will not stick with your program. And remember, light activities, if done daily, can help you become more fit. Build variety into your program. Train with regularity.
Fitness is cumulative.
Increased strength and flexibility result from regular physical activity. Sporadic exercise, especially if intense, can result in injury. Soreness from an intense workout should not last more than 24 hours. If it does, you should re-examine how you are performing the activity and the intensity of your workout. More is not necessarily better.