I am an avid runner and have ran marathons as well as half marathons (usually the Rock and Roll series marathons), best advice is to invest in a HRM, I have a Polar FT 60, it helped me train for my first few because I knew which zone I am in. Now I use it because it helps me count my burned calories while i track my calorie intake. these are really important numbers for continual weight loss! Hope that helps!