6.8km in 40 mins + 20 mins workout

Today I decided to do cardio + a workout. The run was much faster than my previous one and I managed to do 6.8km in 40 mins which make my rate 5.88mins/km. I'm sure that I can reduce that to 5.55 for my 10km run in 1 month's time. I also did a 20 min workout with pushups, crunches, leg-lifts, lunges and stretches. I'm not doing enough cardio for my own good. I suppose its because I need to get new shoes. I have been using New Balance so far and am very happy with it. I wonder if I should stick to it or buy a new brand. I heard that mizuno and asics are the best for running. But I've not tried them and I'm not sure if I should go for a new brand.