With Exercise, Start Where You Are
If you have fallen out of your activity schedule over the past couple of months, it is time to reset your routine; if you think about exercise in the following way, it will be easier to start and sustain.
For any exercise you do, there is some amount you can complete before you tire out. That amount does not matter. Whether your activity is minutes walking, distance traveled, stairs taken, this is nothing more than your starting point. The most important metric each week is not that number, but how that number changes over time.
To reset your exercise, look for improvement over time. If you do X this week, but then do X + 1 the next time, you win! This sustainable goal allows you to start exactly where you are.