Good news- I still have a much lower resting heart rate from running years ago- I haven't run in about 12 years due to compounding injuries- but the benefits are still there. It's finally starting to go up to normal, but it used to freak nurses and doctors when they'd measure it.
I feel like gains from running stay with you a long time also. Once I was a runner, it was so much easier to go back to running five miles easily if I ever quit and went back to it. I think there is some body and muscle memory that remains.
I think I've read that true endurance and strength of your heart is built running over one hour at a time and in my experience that seems true. Our bodies are so lazy and don't make changes until they absolutely have to. I run 7 miles every Sunday or so. I based that number on that and this paper which shows mortality vs. distance run per week. People that run too much have mortality like a sedentary person!
