Feel so out of shape

After seeing Grow with Jo suggested here I decided to try one of her 12 minute workouts and I seriously feel so out of shape. It was only 12 minutes and I am so tired and sore after. How do I start getting back into shape without feeling so tired quickly?

I’m walking 8k steps a day, taking 10 flights of stairs and trying to get activity in however I can, but it’s disheartening that a few years ago I was able to squat 130 lbs and now body weight alone is killing me