One thing I'd really echo here is the emphasis on "realistic". And not "realistic for somebody else" or "realistic for your imagined optimal self" but realistic

One thing I'd really echo here is the emphasis on "realistic". And not "realistic for somebody else" or "realistic for your imagined optimal self" but realistic given not just your current fitness level but also your current level of skill in following through. As an example, travel and illness threw me off my regular schedule this summer. One I haven't exercised for a while I get kinda miserable, so exercising seems impossible and I have a hard time getting back to it. I couldn't just set a goal of 3 solid workouts per week; I would have failed. What I picked was a daily streak of at least 10 minutes of elevated heart rate (zone 2 or better) per day. A short, brisk walk would be enough to qualify. The first days sucked and I had a couple false starts, but yesterday was streak day 18, and now going out for a 3-mile run or doing 30 minutes on the indoor bike actually feels good. So now I'm ready to commit to something more serious, but it took a ramp-up to get my brain ready for 3 real workouts per week.