Hey Quercus,
I know on my watch I feel like the watch overestimates my calorie burn, I think possibly because of the extra weight. I dunno. There is this site which likely uses a different calculation:
I entered in a rough guess of your stats (38, 318, 60 minutes at 140, male) and it spit out just about 1,000 calories. The equations are also listed there. Apparently it's all based off a study that was done. FWIW.
Good job on the exercise, and I love my heart rate monitor to death so I hope you enjoy yours. Not sure I could have gotten this far without it, even if I'm not using it quite as much as I used to (after a while you begin to be able to tell what various heart rates feel like). I still always use it on any mountain hikes or anything that's going to push me cardiovascularly so that I know when my HR gets into the high 160s so I can rest.