Preheat oven to 375 degrees. Prepare ham by making several long slits lengthwise down the ham, about 1/2" deep. Place in a large roasting pan.
In a medium bowl, whisk together remaining ingredients (it's okay if it's a little lumpy). Brush about 1/3 of the glaze on the ham. Bake 1 1/2 hours, uncovered, basting several more times with the rest of the glaze. Carve away!
P.S. Horseradish has been around for about 3,000 years, first used by the Egyptians. Back then, it was appointed as one of the 5 bitter herbs Jews were told to eat at Passover. The early Greeks called it an aphrodisiac. When it was first processed in the U.S. in the mid-1800's it was known as 'stingnose'. There are 2 calories per teaspoon. Its biggest fan is probably Dagwood Bumstead. He once said to Blondie in a cartoon strip: "My kingdom for some horseradish!".