Getting fixed on your averages is understandable if it's important to you, but it comes with a negative...you're never happy, always wanting more (I'm talking for myself here)
At 45 or so, I could easily maintain 15 average over 40 miles, modestly rolling countryside, non stop cycling. I usually rode 125 to 150 miles a week.
I wanted to increase my fitness, push myself and one year in particular, worked at upping it. Heaps and heaps of hard work, pushing at every chance saw my average up to 18 mph over 50 or 60 miles...I'd bust a gut and enjoy it...but it got to a point where I suddenly stopped enjoying it and baulked at the idea of hammering myself for 3 hours.
I slowed it back, felt a bit rueful at that loss of peak fitness, but enjoyed myself more.
Ill health and damaged lungs saw it all the disappear for a couple years and at 58, I've just had my first full year back cycling and can still easily average 15...but am struggling to find the time and mojo for 20 mile plus rides. And then I think...what the hell , just ride. And as soon as I get on a bike, that's it, I go as fast as I can. It's just in you, you can't help it