hobkirk, Congrats on your recovery and commitment to getting fit and strong. There have been many good suggestions on how to raise your average speed. I also think some are a little misdirected in that the recommendation is not necessarily focused on your goal but rather on what worked for others.

I believe your question is how “you” can raise your average speed for a 1 hr ride over your current rate. To raise your average speed in the context of your question I believe you will need to accomplish several things to maximize the result. First and foremost you need to raise your work threshold to be able to sustain the necessary watts for 1 hr or longer to average 20 mph. In order to do that your training needs to address endurance and strength improvements. You can also realize improvements by being more aero and efficient in your riding.

Using an online calculator riding a road bike with 23 mm tires, with a rider at 225lbs, and average speed on flat land at 17.0mph it takes 158 watts to ride:
17.0 mph with hands on “tops”
17.7 mph with hands on hoods
19.0 mph with hands in drops
20.5 mph riding in aero bars

Riding the same bike at 225lbs and at 241 watts will result in:
20.0 mph with hands on tops
20.9 mph with hands on hoods
22.6 mph with hands in drops
24.3 mph riding in aero bars

As you can see position on the bike is very important. Guys who do TT’s are very concerned about position. Do you wear loose fitting clothes? Anything flapping steals watts due to increased drag. Tire pressure can also take watts from your effort.

In order to increase your 1 hr watt average you need to build endurance and strength. I don’t believe doing short maximum effort intervals will bring easy results, rather you should do “over/under” intervals that will raise your lactate threshold which is the ability of the body to do sustained hard work. Those intervals could be 5 minutes at 22 mph then 5 minutes at 17 mph times several sets. It might be hard at first to do 5 minutes at 22+ mph, so do 3 minute intervals and increase the interval time when able. Doing those style intervals would require a recovery at 1 to 1. The idea is to be able to go longer and longer while putting out more power than the goal effort. The other component to increase the average speed is to do Z3 HR rides at a constant pace for longer intervals up to 1.5 hrs. During those intervals it is a good opportunity to work on being in the drops for longer and longer amounts of time. When you hit a hill and the speed drops is the time you can get out of the drops to relax and rest the arms and hands. The longer Z3 interval will build up your system to process fat, food calories and energy stored in your blood and organs to supply a consistent energy source. Over time as you do the over/under intervals your Z3 speed will increase. Riding @ Z3 is work and at first there will be muscles crying for a break, keep in mind this is work and they need to obey. Your ability to ride @ Z3 HR will increase and you will be able to creep the HR up to upper Z3 low/ Z4 at times. When you can ride 1hr at that rate you will see big improvements in average speed.

Keep in mind this is a TT type effort to get the average speed up and not a road race where you need to be prepared to go way deep in the pain cave then recover only to do it again and again. TT’ers work steady and even during their efforts/races. They do not attack hills then coast down the back side to recover from the hard effort.