Hello! I am a high school runner (female) with a PR of 19:38. First of all, I'd like to say that a 22:08 vs a 21:00 5k is a pretty large difference, even if you are soloing it. To give you an idea, when I ran a 21:04 5k (on a difficult, hilly course), I was running my 30 minutes tempos at about 7:00 min/mile, which equates to approximately a 21:45 5k... that is when I was doing a 4+ mile tempo. However, it may have been an off day, and the point of this post isn't to lecture on how close or not you are to sub20.
My number 1 tip for sub 20 is train at sub 20. During interval workouts, be sure to be running at least at 6:25 pace, and if you cannot do that cut down the reps or the length. The point is that to run a 5k at 6:25, you must be comfortable (ish) running at that pace. As you get more fit add more reps or length. If you cannot do 3xmile at 6:25, it is very unlikely that you will be able to do 5k at that pace. The last workout I did before the first time I broke 20 (19:54) was a 2x2400 at 6:15 pace. My 4 mile tempos were at about 6:50. My easy runs were almost always 6 miles at 7:45 ish pace (remember easy pace should be easy). That's just to give you an idea. Also add hills if you are running it on a course (I only ran XC5Ks), if you are running on a track, it should be easier to go sub 20 :)