Relating to my other comment under your post, I feel like I am becoming this. I urgently need to stop it and am looking for books on this topics.
e.g. pottery, crossfit, book club. for me, it was bjj, a world of warcraft group, and a "beer club".
regularly watching and chatting in a small twitch stream could be a start, but beware its parasocial nature
solo activities add structure but social bonds reinforce discipline and motivation. "someone will notice my absence".
Someone who relies on you, whatever the context, is some of the greatest motivation out there.
Not kids. Maybe start with a gym or workout buddy. Then work your way up to projects or volunteer work, with people you can't blow off.
Sure, it could be kids, a partner, a spouse, or a friend or family. But it could also be the rest of the team on the weekly bowling league, the puppies at the shelter who need playtime each week with a volunteer human, the community one serves as a volunteer firefighter, the homeless shelter where one helps serve the weekly dinner, the neighbor who needs help with yard upkeep, or any other parts of the village where one lives that relies on you, and makes you feel included, involved, and fulfilled inside by having that purpose.