How do I improve my 4-5km running time?

So I run a 4k in approximately 22 minutes and I'm a guy. Used to be in good shape in highschool 4 years ago but college and my recent desk job took its toll on me. I'm a bit overweight as well. I'm 6 feet and 220 pounds. My roomate runs a 4.5k in 22 minutes and she's a girl but trains alot. I want to improve my 4km run to about 22 minutes but I have no idea how to train to improve on shorter runs where you aren't really training for speed but not really training for endurance either. So what are some things I should go to improve my time other than obviously run every day, are there foods I can eat, do I drink gatorade while running? Should I be running in intervals like fast then slow then fast then slow? Or do I just need to lose weight so It's less energy to run? Any tips at all will help
Answer:

Interval workouts. Speed workouts. Tempo runs. Heart rate monitor training. A simple workout would be: 10 min warm-up run 10x 400M or 5x 800M at faster than 1 mile pace. (2 and 4 minutes rest between each interval respectively) 10 min cool down Throw in an LSD run to lose weight -- for every 1% of body weight you lose, your time will improve by 1%. This does not include the gains obtained by endurance workouts.

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