To second this, I slowly built up doing leg raises/push ups/planks since high school as a habit, I've never skipped more than one day at a time - it's astounding how good of shape I am in now, without ever having to go to a gym or worry about having the time.

Start super small, and increase when it feels too easy. That's all! You can skip every other day if you need, but never twice in a row. Then it becomes a habit in my experience.