Can't get my breath need to train?

Hi, I am 46 overweight with a big pot belly, I smoke, but am determined to give up this time (gave up for 5 months last time, but wife said I was too miserable, so had to go back on) anyway I have a sit down job so get no excercise, I have noticed that when I run even a little way now I am struggling to breathe, so I think its either my heart or my lungs starting to give out, or could it be that I am so overweight and unfit that this is a normal bodily response to lack of excercise? I have today started a little excercise regime in that I jog about 5 mins to a small hill then run up and walk down, I did this three times, each time having to stop to get my breath, and I felt so tired I could have collapsed there and then, is this regime a help or hinderance? and how should / could I improve? Also when I got home I wanted to rest, should I rest or keep active? One other thing, I have heard that one is more prone to get ill after training, should I shower & keep warm or carry on. Thanks!