How to increase speed?

If you do the same thing you've always done, you'll get the same results you always got.

You need to challenge your body if you want it to adapt.

Change it up. Do a long ride once a week. (50? 75? 100 miles?)

Do sprints a couple times a week. Maybe hill sprints one day, mile repeats the other. Different workouts do different things for your strength speed and endurance. Sprints help with strength and push you aerobic threshold higher.

Add in different sports to relieve boredom and give your cycling muscles a breather while still pushing your limits. This is one of the reasons triathlon has become so popular. Brick (bike/run) workouts are one of the most effective sessions I know of. Distances vary depending on what you want to accomplish. Before they closed it off, I used to go to inner ring service road at Fermilab with a group of friends. The circular road about 3 mile circumference (I think). Ride 3 loops, run 1, repeat. 2 or 3 repeats of that at race pace will get your attention. A very effective sprint workout is 10 or 12 repeats of one mile on the bike, followed by running a 400 on the local college track (bike on the road, not the track).

Race. There's nothing like an impending competition for motivation. The people you're "racing" now may not even know they're being raced. If you're not comfortable riding in traffic, consider time trials, "the race of truth". No drafting, and you can't hide from the clock or your competition. You need goals to achieve anything. Racing will help you set your goals.

Silverbullit pretty much hits it on the head. If you want to improve you need a challenge. Bike paths are not a good place to train, unless the path is almost deserted. I only ride bike paths when I am in a relaxed mood, the varied traffic on these routes makes riding fast risky. If you want to improve, start riding with groups, join a club where you can progress from slower to faster groups as you improve. If you only ride about 20 miles, you won't built up endurance so that you can hold a higher speed for extended periods. As was stated, interval training is the way to go to increase top speed. Grinding along at the same speed all the time won't help you get faster. Race riders train at varied speeds, they will ride out doing a long gradual warmup at moderate pace, spend time doing speed work at race speeds, maybe throw in some hill climbs, then maybe head home at a tempo pace, never worrying about their average speed. Most intermediate riders spend too much of their time worrying about the average speed of their rides, riding at the maximum speed they can maintain for extended periods, but never pushing themselves up to their maximum effort, they ride too fast most of the time, but not fast enough all of the time.

A heart rate monitor is a good training tool. I like to go out and climb some pretty steep hills. I go until I get by heart rate to a certain point and can hardy catch my breath and drop back to a lower gear and keep going to maintain that level. You can also go intervals where you ride as fast as you can for 30-90 seconds and then drop back to recover. I have sections of my route that I sprint or try not to drop below a certain speed. You can extend your ride to 25 miles and see an improvement on your regular course.

Your ave. is not that bad, to get better work with a heart rate monitor, you need to find your limit. After you have found your max. HR then you can work on targeting work zones. The end goal is to raise your max. sustainable wattage. There are a few good books on this and recommend getting one as it would be a very long post to give you all the information you need.