[Vmysobfi]

At this rate "target heart rate" is a bad word. Don't even think in those terms. You said it yourself, your body is not use to any stress at all, so any raise it heart rate, will actually do you good. I would probably forget about any and all numbers associated with exercise right now, and just get out there and do what ever I could for as long as I could. Listen to your body. But don't be too quick to give up either. I find that when it's been a long time since I've done a certain activity, it takes me about 10 minutes for my muscles to tighten up and then feel a burn... if I push through that, I can go for hours.. but make a vow to yourself to do what you can, each time, and then go a little further.. then next time,, increase just a little.

{I would also start with walking,, bike riding may be a little too much right now}