Speed...

Why do you think even pace would be a good thing?

On a solo ride, the fastest overall time - and the highest average speed - is from riding as fast as you CAN, all the time.
This automatically means you'll be a bit slower on every incline, every headwind section etc.
Or simply when you're feeling tired.
And that's just how it is.
You wouldn't be making yourself any favors by running past your lactic threshold(="ability"😉 only to maintain an even pace. Whatever you'd gain from climbing harder than you should, you'd lose a lot more from recovering on the next flat section.
A heart rate monitor or a power meter is a better way of measuring if you're riding OK than looking at mph.
Speed is really only important when measuring yourself against others.

For starters, there is no such thing as a flat road with no wind or a constant wind. Even roads that appear flat have small changes in grade and grade has a huge effect on speed. As to wind, even a wind perceived to be fairly steady is rarely steady at all. Second, as dabac said, it's rarely an objective to ride at a constant speed. Constant power or constant heart rate, yes, but constant speed, no.

thanks you for share!