So, you've been training for just over a year and got to 18:22, and wonder if you could go under 18?
Obviously, the answer is yes!
ANY organized training program should get you there, really. It sounds like you have just been going for runs 5-6 days a week, which is great.
Make sure that you consistently have the following components in all or most weeks:
- Intervals at or around 5k pace. I guess 5:50 pace is the right ballpark for you. 800s, 1000s, even some quicker 400s. Do around 5k worth in a session. 1:00-1:30 rest depending on the distance and your aerobic fitness.
- Threshold work. I don't know much about your running, but for you this will probably be around 6:15 per mile. Do four mile tempos at that pace. Do six mile tempos a bit slower. Do 3x2 or 4x1.5 or 6x1 mile a bit quicker. Just hit it for 30-40 minutes each week. (Some 10k races won't hurt, either.)
- Long run. 10 miles is fine for your purposes in a 5K.
Make sure these are in your schedule, just run comfortably the other days, and take one non-running day per week if you like.
I started running late like you, and trained myself down to 16-minute 5Ks on these loose and well-worn principles. Stay healthy, and you will absolutely break 18.