If you really want to better your endurance and overall physical fitness, cross-train. You don’t get the time to rebuild muscle if you run every day, but if you do other exercises on your off days that use different major muscle groups (rowing, swimming, cycling), you still get a good cardiovascular workout. Even when you cross-train, you shouldn’t be working out every single day anyway.

Aside from just running, you are doing weight bearing exercises as well, yes? If you aren’t, you should be. And you should be working all opposing muscle groups during said weight bearing exercises.

And quit smoking. Not like I will, but I had to say it anyway.