To be a runner, you must: 1) eat right, 2) lower your body fat, 3) avoid smoking, 4) sleep well, and 5) exercise regularly. Be a runner!
Don't compare yourself to faster friends. We all possess different levels of ability and training. Focus on being the best you, you can be.
Consistency! Regardless of fitness level maintain a base of training. Run regularly. No gaps. Do an occasional race. Never get out of shape.
Don't measure exercise only by heart rate or calories burned or miles run or pace or any e-means. How about flowers sniffed? Sights seen?