I have had the Apple Watch Sport series 2 38mm for a couple months now and when I'm running without the iPhone I get about 25% drain per hour from the watch with everything on including listening to Bluetooth headphones. Does this sound about right for the 38mm? I'm a rather average/slower marathoner and I usually finish in about 4hrs 30minutes. I'm wondering if the watch would last or if I would have to turn off the heart rate off or if it will even make it that long at all!?