how do I fix this and not lose my fitness level?

To keep conditioning up you might want to try non-running stuff. The first thing that came to my mind was Tabata Thrusters. They are basically a squat where you hold some light dumbells at your shoulders. When you come up from the squat press them over head and lower them, then squat again. Seems simple enough, but then you add to it using the Tabata protocol which is IIRC 8 rounds of 20 seconds doing this exercise as fast as you can with good form, followed by 10 seconds of rest. Repeat. This will give a 4 minute workout, not including warm-up and cool-down time, that will give lots of conditioning. Here is a video of someone doing them http://www.youtube.com/watch?v=peLFTePbfDQ

It sounds easy, but this is one of the hardest workouts I have ever done. I am completely exhausted by it afterward. The goal is to do the repetitions both at a speed and weight that allows you to do about the same amount every interval. If the numbers drop off a lot near the final intervals you are either going to fast or using too much weight. Doing these more than three times a week is probably more than you should do. You need at least a days rest between sessions.

It sounds like you're in pretty good shape, physically. I'm assuming that you haven't torn a muscle. A common reason for cramps of any kind could mean that you're not getting enough carbs. for your higher level of activity; &/or this could also be a problem with not enough electrolytes, especially potassium. During exercise, your muscles burn calories from any source; if you run out of readily available blood glucose, you're body will use the Krebs Cycle to make glucose from fat, or from muscle if not enough fat is available. Some people can feel light-headed, dizzy, or a little foggy when this happens; a few feel energized. Another side effect of the Krebs Cycle can also be cramping from the "junk" byproducts while in the muscle that are normally excreted by kidneys & perspiration. Perspiring makes you lose electrolytes and water, and so does flushing "junk" through the kidneys. There is a reason that all the sports drinks available OTC include readily absorbed primary sugar of some type as well as electrolytes. Try using sports drinks and see if there's an improvement (actually, reduction) in your shin splints. Of course, you'll need to include it in your diet plan. I hope this helps!