So bored wrote:
... Can't believe it when I see masters my age still crushing it. I've lost nearly all motivation, am tired all the time, ...
I used to be like you. Got home from work. Sat on the couch. Too tired to get up. Couldn't believe it when I saw people my age still running, biking, and driving around with kayaks on their roofs. I had a knee injury and couldn't run anymore.
After complaining about my knee injury to my older brother, and how I probably needed some kind of surgery for it to heal, he told me: "Just ignore it. Go running anyhow. It'll probably just heal itself."
I knew that wasn't going to happen, but I also knew that the only way to prove it was to try running everyday. So I started running and my knee hurt. I didn't care, I kept going. It got a bit better and somewhat tolerable. Then I signed up for a 10k race. It seemed crazy, because it was so far and I was so fat. Finished that and signed up for a half marathon. Couldn't believe I signed up. Couldn't believe I finished. Couldn't get out of bed the next day and had to take a day off work. Then in a rash decision, I signed up for a marathon. Ran it, finished and signed up for another. That was 13 years ago, and I have run 40 marathons since then.
So, just set your excuses aside, start running more, if it's dark by a light for your cap, and just keep going.