Fitness is not simply cardio.

There's a great Men's Journal article, featured a few times on HN, "Everything You Know About Fitness is a Lie".

Read it. Several times.

https://web.archive.org/web/20110130195636/http://www.mensjo...

(Earlier discussion: https://news.ycombinator.com/item?id=4971196 and https://news.ycombinator.com/item?id=2287213, particularly.)

Your body is an adaptive system, it responds to stimulus, given nutrition, rest/recovery, and other parameters (of which stress is a major element).

Cardio works your heart. That's useful, but there's more to your body than that.

Strength training works your muscles, building strength, endurance, power, and mass.

There are skills, fine-motor control, coordination, and other exercise modalities, with their own specific results.

Training is a stimulus which achieves a specific result, preferably one you've identified as a goal.

All said, 20 minutes of daily walking ... isn't a lot. It's better than nothing. There are other things you can do. You don't need a lot of equipment or space (though as with many things, the right tools for the job help).

Understanding how your body responds to what stimulus, under what circumstances, and roughly how quickly or slowly, helps a lot.

A modest, consistent, balanced, and goal-appropriate programme can be absolutely life-changing. It's valuable at any age -- what you do when you're young will deliver returns in your old age (the more so if you keep it up). Training as an older adult is also a major quality-of-life determinant.

There's no One Perfect Exercise. A balance of modalities (cardio, strength, flexibility, skills) is generally preferable. Starting out, virtually anything is better than nothing. As you advance, you'll find you need to make more specific tweaks to continue progress (should you want to).

And once you've gotten yourself beyond baseline, a small amount of work goes a long way. Clarence Bass, now well into his 80s, maintains an impressive physique based on two workouts a week -- one lifting session, one cardio.

https://www.cbass.com

There's a ton of bogus advice, but also quality information readily found. The basics are simple, though the work isn't always easy.

Reddit's /r/fitness (or /r/XXfitness for women) is actually quite good, both in discussion and its wiki. Strongly recommended:

https://old.reddit.com/r/fitness

https://old.reddit.com/r/fitness/wiki

https://old.reddit.com/r/xxfitness

https://old.reddit.com/r/xxfitness/wiki