Advice for a middle-age, moderately pc knowledgeable person to finally switch to or become proficient with Linux?

As an alternative (and since you need to keep Windows running for now), have you considered downloading VirtualBox and installing linux thee on your desktop? There's a couple of really good reasons behind this... First off, you don't have to mess with switching back and forth when dual-booting, but it also gives you the ability to play with some different distributions and find something that feels more comfortable without having to trash a machine and continuously starting over. You can even load up multiple distros at the same time on your Windows desktop and compare them.

Now for really getting going with linux... It's easier if you don't expect yourself to figure it all out at once. Pick a daily task, like reading your email. Maybe you already use something like Thunderbird, so that's an easy switch. Just shut it down on Windows and start using the linux screen for this every time. Web browsing might also be an easy switch, and you already mentioned you use Gimp. Have you played around with different desktops yet? With linux you can install several and select one of them when you log in, so maybe try KDE, Gnome, or Mate (this one is my favorite because it's fairly light on my older system but still lets me configure a lot). What else can you dive in to? How about configuring your login screen for different options like showing available users or automatically signing you in when you boot up the linux system in virtualbox? There's a lot of configuration you can do by editing files in the /etc/ folder so it pays to get familiar with that aspect of things.

Once you think maybe you found a distro or desktop you like, consider what other things you can do to really start getting familiar with linux. You could change your default shell, or your default command line editor. What about setting up a local DNS cache or maybe your own email relay? Maybe even set up your own web server and database, and use them to develop web pages locally. The point is, once you realize there's no limitations in what you now have available to you, your brain starts thinking about all the things you could do -- and that's where you really start learning how to work with linux. Sure you need to use online guides the first time you set up a web server or something else. Sure you'll have to continue to rely on the web for some time, but as you work through various projects you'll start to remember where you found things, and you'll start to see how they connect to other things, and one day that thing you've been re-typing from your notes suddenly clicks and you understand WHAT that thing was actually doing.

It's never an easy process to start something new. The first time somebody gave me a bunch of old PC parts and I powered up a machine on my waterbed, I looked at the blinking cursor and thought "now what?" (Yeah he didn't even give me a DOS boot floppy!) But here we are thirty-some years later and I'm running servers, troubleshooting multiple operating systems at work, and doing pretty much anything I want. Linux was a complete change of gears but it was totally worth taking the time to figure it out, you just need to work on one piece at a time.