Sarah is participating in a charity marathon that is 26.2 miles long. She runs the first 10 miles at a steady pace. For the next 8 miles, she slows down and runs at half the speed, taking twice as long. She then walks the remaining distance. If she has 2.5 miles left to walk, how far did she run at half speed? Let's think step by step and output the final answer within \boxed{}.
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