This is very true. Long ago I started biking to work, and biking really hard - basically making the best time I could.

The first few weeks, at the end of a 10 minute sprint, my heart would be racing and I would be out of breath for 5 minutes after arrival.

I was amazed after 2 months of doing the same thing I wasn't out of breath, it took maybe 1 minute for my heart rate to slow back down.

Define substantial. I find that incredible.

EDIT: Now that I've seen the replies, I realize that this is from a very low baseline. I'm personally going to the gym four to five days a week already and my own cardiovascular fitness has not seen much of an improvement in many months.

Not a source, but it tracks for me. Walk, jog, or run daily and you will have massive CV improvement from baseline in 3 months. people can and do go from being exhausted walking to their car to being capable running many miles in an incredibly short time.

The typical human body is very responsive when pushed.

The heart, lungs, and muscles get strong fast. But the bones and tendons take more time. It's best to give them time so you don't wind up with shin splints, etc.

I went from having not run in years to running a 5k in about 2 months. I was obese when I hit that goal, though I recently graduated to being merely overweight.

My VO2 max as reported by my Apple Watch has been making steady gains, though I'm still far below average for my age.

I didn't train hard, just consistently.

It’s true. Between myself and some close friends I have seen the following in ~6 months: - unable to run -> run a 10k - increase of ~50% in a all out 20 minute cycling test - max mile time 9min->6:30min

The human body can make incredible cardiovascular leaps when starting from a relatively low point

One example: https://en.wikipedia.org/wiki/Couch_to_5K

9 weeks. I have basic fitness but have never been close to completing a 5km run (I think I finally realised I have breathing issues - seeing an ENT soon).

n=1, but I can personally attest to going from average sedentary 29 year old to marathon running shape in roughly 3 weeks of continuous hiking 8 to 10 hours a day in the Appalachian mountains with a pack that was about 35% of my body weight.

I have a hard time believing that a truly sedentary person could do that.

I wasn't overweight, I just sat 8+ hours per day. If you are interested, here is the photo journal https://www.facebook.com/media/set/?set=a.1420283804872269&t...

the only way I was able to improve mine was to use interval training to get my heart rate up. It took a few months but going from a walk to sprint levels on my elliptical did wonders after a while. Observing my heart rate carefully of course.

It's the same with lifting and adding muscle mass.

If you've never lifted before and you hit the gym consistently and have the proper diet, you will get some serious "newb gains." The first year can be amazing.

But if you've lifted seriously for a decade, it's totally different. Muscle mass is very hard to add now.

Cardio gains from a coach potato introducing a serious routine will be incredible. I went from not being able to run more than 30 seconds to running a 5k in 4-5 months, and that's without pushing myself extremely hard.

Why would you want to add muscle mass when you've lifted seriously for a decade?

Many bodybuilders are aiming to add as much muscle mass as their genetic potential allows, which can take more than a decade.

The point is about how the body adapts.