When running you increase your time or distance by 10% a week at most - build up gradually. Your body will get used to doing one particular exercise but shake it up a bit, you should still ache occasionally, it suggests you are still working hard!
Also if you're pretty new to exercise then you need to have more rest days than so done who has been exercising regularly. Rest doesn't mean sitting on our butt all day, keep moving - walking or swimming are usually pretty good "rest day" activities. Also if you have a niggle, be very careful it doesn't turn into an injury, rest and see if it goes quickly.
Do people think it's a good idea to do hard exercise every day, or is it better to have a rest day and do it every other day? I am not trying to lose weight, no particular fitness goal (apart from getting fitter!) I'm just wondering if it's better for joints and muscle recovery to have that rest day, or if I'm just being lazy