Refusal of shoe exchange

On the 27th Sept, I bought an Adidas trail running shoe fro Cape Union Mart. I ran with the Adidas trail shoe at a 27kms trail race which was on the 14 Oct. During the race,at 5kms I experienced some discomfort on my left big toe and changed the shoe to another pair and ran with it till I finished the race. At the finish I realised that my big toe had bled internally. I then returned the Adidas to the store requesting an exchange for a bigger size which I was refused. The manager reason was that I had ‘used’ the shoe and I was supposed to walk around the house to test if its the right size. My question to her was, the shoe obviously does fit me for walking around but the shoe is meant for trail running which is exactly why I bought it in the first place. How then was I supposed to test a running shoe without running using it? I am so dissappointed with service from this store