It appears that we are again confusing cruising speed with average speed. The original question was clear on this.

My answer is complicated. I don't think it is a good idea to judge yourself by the exact speed you can maintain on windless flat conditions. Since you are new to this, you have to give yourself time to adapt into the kind of cyclist that you will eventually become.

The human body is amazingly adaptable, and with the right kind of training you can continue to build your power almost indefinitely (until you reach some critical limits). The process is long though, as the muscular adaptation system works slowly and the gains happen over years, not over weeks. The training plans have to be designed to operate in smaller time frames though. Striving for any tiny increase in your performance over the short term will eventually yield large increases over the long term.

If you are making 15mph now and it feels like an effort, then you need to build based on that, not based on the kind of speed that someone with ten years of racing experience has.