There's no hurry to buy a watch. When you start running regularly (not less than 3-5 times a week), you should make more theoretical preparations, such as what wrong running postures are, how to avoid knee pain and iliotibial tract pain.
If you just want to run the horse completely at once, run regularly and gradually increase the distance. The key is to form habits and turn persistence into habits. The fast pace can be seen from many mobile apps. Want to see the heart rate, you can match a sports bracelet, and it will be solved in tens to two or three hundred. Don't play horse racing with tickets. If you haven't practiced or even run more than ten kilometers, just to make a circle of friends, you have to challenge your limits.
In addition, in addition to pace and heart rate (also the most primary indicators), learn the concepts of maximum heart rate, heart rate interval, easy running, lactate threshold and so on. At this time, if you want to pursue higher grades and improve by scientific training, buy a watch.