I've started working out on the tredmill at 12km/hr for 30 minutes almost every day. For the last 3 minutes I kick it up to 14-15km/hr. Is this a good workout or am I better to run on an incline but slower?
Running flat will tend to train your stamina while running on an incline will help to increase your leg endurance and strength. So the answer depends on your running goals.