Sounds like you have developed a repetitive strain injury. Your best bet is to go to www.julstro.com and post on their forum (authors of The Pain-Free Triathlete). They work closely with triathletes who develop these kinds of injuries.

What may be happening is that your muscles are either going into spasm, or simply shortening up, due to your bike work. If you think about it, the muscles of the leg are quite confined in their range of motion when doing hours of peddling. The shortening of the muscle, combined with the lactic acid build-up also causes pulling on the tendons that attach at the ankle or foot. That is why you will also feel pain there. I developed quite a case of Achilles Tendonitis from biking, but when on the bike, was about the only time it didn't bother me. The pain was in the heel. The problem was in my calf muscles. The web site mentioned above gives a good explanation of this in their "muscles and pain" section.

I think you will find your answer there. Good luck.

What side of the tibia is the pain on? Inside or outside of the leg? What part of the foot/ankle does the pain radiate through? When do you get the pain? Cycling or running? Does it get worse during the course of exercise, or get better when you warm-up?

You've got to give the "I wish I were a Dr" or "Oh, Ive had that one" community more to work with.

Keep in mind your bike fit, even the pedal settings can have a major inpact on your body's mechanics. Do something a bit too strenuous over and over again, and you will develop the repetitive strain injury mentioned above. So you may have to change up your bike config along with getting some bodywork (physi/massage/etc.) done.

ditto on the www.julstro.com advice from Train Wreck. He gave me the same advice when my shins started killing me. I followed it, and now I'm pain-free. Good stuff.