Wearing the high-tech shoes, Candace can hike 6 mph * 2 = <<6*2=12>>12 mph for the first two hours.
Then she gets one blister, so her speed drops to 12 mph - 2 mph = <<12-2=10>>10 mph for the next two hours of the hike.
After that time Candace gets another blister, but 2 hours + 2 hours = <<4=4>>4 hours, so the hike is over and we don't care about her speed anymore.
Candace spends an equal amount of time walking at 12 mph and 10 mph, so we can average the two speeds to find an overall speed of (12 mph + 10 mph)/2 = <<(12+10)/2=11>>11 mph.
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