Gabe Mirkin, M.D.
Competitive athletes train by taking a very hard workout, having sore muscles on the next day, and then they take easy workouts until their muscle stop feeling sore. If you exercise just for fitness, you probably don't need to train that way. To gain strength, speed and endurance, you have to stress your body by taking a hard workout. This damages muscles and they feel sore on the next day. If you exercise intensely when your muscles are sore, you increase your risk for injury, but if you exercise at a leisurely pace each day until the soreness goes away, your muscles will heal and be stronger then they were before a hard workout. Doing nothing on the days after a hard workout can help your muscles heal faster, but your muscles will become stronger and better able to withstand hard exercise if you exercise at a leisurely pace on the days that your muscles feel sore. Competitive runners run very fast only one to three times a week. Competitive weight lifters lift very heavy weights with the same muscle groups only once every one or two weeks.
If you don't compete and don't need to improve, you don't need to stress and recover. You can take the same easy workouts every day, provided that you stop exercising when your muscles feel heavy and hurt or start to burn. If you want to improve however, you have to exercise until your muscles hurt and burn once or twice a week and the go easy in future workouts until the soreness disappears.