I would think a seasoned athlete would be around 70-80 percent. I don't know my RHR, but it usually is around 50-55 when I'm deconditioned (which I still consider myself). I was low 40s when I last ran a marathon. Using the algorithm (30 yrs old) and I tend to be around 165-175 when I finish my run (and feel like I'm going to die), then 170 - 50 = 120 which is 85% intensity. Yikes. Of course, that's toward the end of my bout when I would expect it to peak. Regardless, I think I'm on the higher end (75-80). When I'm not deconditioned, I'll probably run lighter. Right now that would be at 5.5 mph (usually do 6.0 right now), but I want my marathon pace to be my "easy" intensity. But this is just me. Everyone is different and it changes depending on their fitness and ability. Running is always hard for me. I'm a big guy. Even when I ran the marathon, a lot of it was high intensity, but being a big guy I had a good ability to deal with higher intensity workouts, which kept me going. That is ultimately what someone who competes trains for: to be able to push themselves to the limits of their aerobic abilities while still being able to not tire out. The alternative is just to work well within your aerobic energy system (full oxygen intake) and just have the body mechanics capable of going the duration. I mean, anyone should be able to walk a marathon without significant damage to their body! It is an achievement to do it, but doing it in under 4 hours is another beast. So goals are everything.