Buying good running shoes is so important.
Let me repeat that: Buy. Good. Running. Shoes. I know they are a little expensive, but it's one investment you don't want to skimp on if you're aiming for the finish line.
I, unfortunately, learned this lesson the hard way. My trusty running shoes had been discontinued and when they became too worn halfway through my training, I was too stubborn to try out a new pair and I found myself suffering from shin splints. It got so bad that I had to take four weeks off of running leading up to the race. I finally bought a new pair of running shoes, just one long run before race day and magically, my shins were fine. Missing four weeks at the tail end of training was devastating for my performance (as was running a race in shoes that hadn't been broken in), and it could have completely been avoided.