You mentioned in earlier post that you've returned to running recently. Give your body time to adjust to the demands. Running every alternate day is a good start and also warm up properly. Maybe walk first 5 min, stretch gently, have a run and stretch after wards whilst body is still warm. Do not bounce the stretch and do not over-stretch. Never stretch when body is cold and unprepared.
On the alternate days, swim, cycle, walk, or do conditioning exercises. Also helps physically and mentally to mix up the running. One long run per week, and a couple of moderate or shorter ones. Mix up the pace when you feel up to it. Do fartlek (speed play) once per week where you run steady pace for say 5 min and then up the pace for 30-60 secs (or between landmarks such as light posts, street corners, etc) for several repeats. It can enhance C/V fitness and combat the boredom and routine that runners can sometimes experience.
My caveat is that this is purely my opinion; not specific advice.. W2W, not sure of your age, if you're new to this and concerned, have you had a medical check-up lately? Careful not to over-train at outset to reduce chance of injury and other complications.
