I don't pay much attention to heart rate - I am relatively alone in that regard. I've worn a HR monitor for certain tests, but it doesn't do anything for me on a regular basis. Some people get a lot out of it, and that's cool. My wife does, and she has really cool charts that correlate speed/watts/incline with HR.
It seems that 90% of people agree at this point that you need "a lot" of relatively low intensity stuff, as mtwarden pointed out. I think something that gets you into the "feels like you are working a little bit" point, (maybe around 120bpm?) is a good target. Not strolling, but not speed walking. 4-6 hours of that a week, and I would bet you see some differences in a month or so. The key is that you keep doing it, whatever it is, and that is the stumbling block for most, myself included. You've got a big advantage in that you actually move around more in a day than most if you're like any farmer I ever met.
If there's any way to get away from the treadmill, I'd work on that, but that's a personal preference.
