In reply to andic: It could be a build up of fluid in the tissues of the lower leg. I suffered similar symptoms last year and a good sports massage got rid of it, bloody hurt at the time though.
Might be worth biting down on something and trying some self massage of the area as this will help clear any excess fluid that may have accumulated because of your increase in running.
I'm sure you don't need anyone to tell you that rapid increases in distance is a sure fire way to pick up injuries. So no more than a 10% increase each week is advisable, that's 10% over the week not 10% on each run.
Saying all of this I'm a runner not a professional and can only go on the brief description you've given.
If in doubt go see a sports therapist.
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