Preheat oven to 375-degrees.
Pat the roast dry with paper towels.
Place the roast on a baking sheet (or roasting pan) and rub it well on all sides with the salt and pepper.
Allow the roast to sit for 20-30 minutes to allow the seasonings to permeate the meat.
Meanwhile, in a medium bowl stir together the mayonnaise, mustard, horseradish, thyme, rosemary, and garlic powder.
Brush the roast well on all sides with the mayo mixture.
Place the roast in the preheated oven and cook for approximately 1½ - 2 hours, or until an instant-read meat thermometer reads 7-10 degrees below your desired doneness.
Remove the roast from the oven and cover with foil.
Allow the roast beast to sit, covered, at least 10-15 minutes before carving.