Have you changed where you run? If you started running on a new surface (especially concrete) your legs won't be used to the change. Concrete is not very good for your knees.
Well before i use to run like 1 maybe 2 times a week now i run like 5 times a week i cant play sports anymore without my legs hurting it hurts around the shin and calf area i think im going to go see a doctor i love running and i don't want to stop its a stress reliever (i could be also using the wrong type of shoes i just recently bough nikes but i havent tried them out yet tommorow im waking up at 4:30 in the morning and taking a 3 mile run lets see how that turns out)
If it's hurting your legs then I'd suggest you should try cycling. It's still consider as cardio exercise. Furthermore, if you wish to use those leg muscles to the max, you could try cycle on those static bicycles you can find in gymnasium. It won't hurt your knee as bad as when you run on concrete or any other surface.
--------------------------------------------------------------------------- ~ Change takes time, not at that time ~
i think candles and wine and a bathtub sounds better. 
(i could be also using the wrong type of shoes i just recently bough nikes but i havent tried them out yet tommorow im waking up at 4:30 in the morning and taking a 3 mile run lets see how that turns out)
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