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.
The typical human body is very responsive when pushed.