I'm male, 26 years old, 5'7" and 146 lbs. After walking a mile, my heart rate was 80ish bpm, I sat down and 20 minutes later it's 56 bpm. Numbers ok?

Exercise buff. You're in good shape. Keep it up.
Yea. Yes. That sounds good. The better shape u are in the more you will be able to exercise and the less your heart rate will need to increase. Try to increase your fitness by walking further and faster over time !
Family practice . It is probable normal for you and evidence that you have a healthy heart.