I have a close connection with running. I stopped for a while due reasons and came back to it. Now doing lots of running on foot and also treadmill as well. Running is actually a very sensitive matter. Whether you are a runner or doing any kind of sport related to heavy usage of foot. Generally, the right surface, the right shoe, and the right way of warming and activity and some other factors are crucial to any runner. Now,
-When you leave it for a while, it gets too much more important, cause your body left it out and you need to return to it slowly and step by step. When you do run, the body's weight is pressuring the shin and knees. Your shin must be exercised slowly and get in form so it could take the pressure of running. So you can't run a very long long mile with the start of your running (after a while) you need to train constantly in small milestones and increase the milestone slowly so the shins become stronger and stronger. So choosing the right shoe and the right surface can secure your foot to not get hurt that easily. A good warming is too important as well.
-It can get really bad if you don't take care of the points that i've mentioned. Also the style of running must be controlled. You should make sure to not do much of "Over-Striding" because it gives the whole pressure to the front bones and muscles of shin. So it can really hurt over long time and if the body be out of form, it can makes you effected. There's a syndrome and injury called "Medial Tibial Syndrome" "Shin Splint" which is bone and muscle injury, a very sensitive injury which occurs when you do a long or constant running without taking care of the healthy points as i said before. Or when you do not rest your feet and practice too hard. Here's the link where you can read about this injury
http://www.chirunning.com/blog/entry/the-whole-story-on-shin-splints
I've inured by this Shin splint months ago due too much pressure cause i was furious to get back in form again after i've been away and i had slow down for a month to heal up completely.
-About the improvement, it's just you have to go slowly and get use to lots of running and take care of the points i mentioned!! And then make sure of the strength of your feet. You can work on the your shins and thighs due work out and make sure you have enough blood cells (Marrow) and calcium and iron in your body which helps to stabilize the strength inside your body.
I guess that you've heard like Soccer players (or any runner activity) their sexual life get controlled as well, with each release of sperm, amount of Calcium get lost out of the body as well. Now if you lose lots of Calcium due different reasons and you don't replace it with nutrition and pills, your foot gets weak and your quality will be lower and then the risk of your injury will be so much higher.