The new toughness training in sports is a good book. The book is a little dated, but still quality.
Toughness is defined as the ability to resist stress (such as the stress caused by physical exertion).
The core premise of the book is that a regimen of progressive stress and rest is the path to toughness (increased physical endurance). Too much stress, physical or mental, requires increased rest, therefore you must experience some stress, but not excessive stress. This means finding the stress <-> rest equilibrium that results in the most benefit.
If you're asking for motivation, my only personal advice, having done this a couple times now, is that you must change your environment. Move to a city and give up your car, it will happen with near zero effort. I found walking to my favorite cafe and then my favorite view to be rewarding and self reinforcing.
You should also set your expectations appropriately. It takes 2-3 months of huffing and puffing before you feel much progress, and then one day, you realize "that wasn't so bad," and you will see your body has adapted.
Also, when people want to feel healthier, they often focus on movement but ignore diet. After a few months focusing on fitness, you start to understand that diet is 90% of the work and the physical activity itself is probably closer to 10%. That's if you want to lose weight or build muscle and it's especially true if you want to build muscle.
I also recommend getting a trainer for resistance training for at least 3 months. It is expensive but worth it. It allows you to outsource many different problems to someone else and lets you focus on the physical work directly.