Regardless of time/distance of the workout, it is always a good idea to train your body and mind to finish strong. Your goal is to run the first 2/3 of your workout at a moderate pace then spend the last 1/3 at race pace or faster. Example: 45 minute run. 34 minutes at moderate pace then 11 minutes at race pace.
Many athletes are interested in increasing their speed/average pace. To achieve this workout, start at a moderate to relaxed pace to warm up then spend the middle of the run in a tempo/race pace. Finish with a relaxed or moderate pace. Example: 45 minute run. 10 minutes warm up then increase pace to feel uncomfortable for 30 minutes. Finish with 5 minutes at an easy pace to cool down.
Intervals are an excellent method to burn calories and increase pace. Generally, intervals are performed at a 1:2 ratio of work to recovery. Interval runs can be performed on a track using distance or using time on the road/trail. Work pace should feel uncomfortably fast while the recovery pace is either relaxed pace or walking. Reducing the walking during recovery will increase the calorie burn and the amount of work accomplished. Example: 45 minute run. 10 minutes to warm up at an easy pace. Five rounds of 2 minutes at an uncomfortable pace: 4 minutes recovery. Finish with 5 minutes at a relaxed pace to cool down.