I recommend the use of a heart rate monitor for when you are running, particularly in the early stages when you are still conditioning your body to a different type of exercise. It is easy to overdo it trying to maintain a steady pace when gradient/headwind are pushing your heart rate too high, that leaves you exhausted and very sore. Plus it is another gadget to play with that outputs statistics and has online software to “investigate”, I am amazed you haven’t bought one already to be honest!