Howitt44

I am overweight and relatively new to the work-out world. I've worked out sporadically over the years , but now I am determined to be consistent. I think that is really the only way to see results. My question is my heart rate. I can walk on the treadmill at 3.5 miles per hour and my heart rate will go up to 140 bpm if I swing my arms and go down to 125 or so if I hold on. I've been told that at my age my heart rate should stay around 130 and 140. I get real discouraged thinking why are other people able to jog and still not get their heart rate up? Well, I know its because they are in shape and I'm not. But is my case common for someone overweight and out of shape? I have people tell me , you need to incline or jog, but shouldn't I work out at my target heart rate and not worrry so much about what I'm doing as long as I keep my heart rate up there for at least 30 minutes? I've been going everyday. I walk for 30 minutes and keep my heart rate at 140. Then I lift weights which also keeps my heart rate up. So my total time at the gym is about 50 minutes all the while keeping my heart at 140 bpm. I hope in time my heart rate will come down and I can do more. Thanks for any suggestions.
Cindy