Find something you love doing and do it - might be running, might be road riding, might be full contact tiddlywinks. Or tweak what you do - get a cross bike and ride back lanes and easy trails, sign up for Strava and go hunting for fast times, play golf if you get it. Or go to the gym.
Running's ace on dark, wet, loathsome days from Mordor when you can't face cleaning your bike and a washing machine full of ride kit. Low hassle, good brain time, effective training too.
But whatever you do, make it something you really enjoy doing or you simply won't stick with it. If you're wired that way and it helps, set some goals for next year: might be an event or a riding holiday or a particular time around a familiar loop, whatever, just pick something that's challenging but achievable and use it as a bit of a carrot.