HI there

Muscle soreness and stiffness is just caused by muscle damage, very microscopically. Normally what happens is that the muscle gets stronger, or the weak fibres are replaced by stronger ones, and you gradually get less and less stiff after exercise. Therefore, it might just be a case of being more consistent in your running. I don't know what your training has consisted of, but if you run every third or fourth day, or if you take a week in between bouts of runs, then you will find that this stiffness never goes away completely, because the training stimulus is so spread out, and you don't adapt quite like you should. If you are running every day, and you've been getting stiff for a number of weeks, then there is a chance that you are not adapting normally to the training load, and there are some medical conditions that cause this. This would be something to look into much later perhaps. However, for now, I would say just keep at it, stretch regularly, and get some consistency in the training if you haven't already been doing that.

Goodluck