I was thinking about what I would tell myself in week 1 with the benefit of hindsight.
1. Jelly legs / legs like lead weights.
Lactic acid builds up in the leg muscles as we run. As we get fitter our muscles process it more efficiently and get rid of it quickly.
But at first we produce more acid than our tired old muscles can deal with. This acid is what causes legs to feel like jelly or like lead.
The solution is simply to run more slowly so that you give your muscles time to get rid of the lactic acid.
2. Lungs burning
Muscles need oxygen to burn energy. If your body demands more oxygen than your lungs can give then then you feel terrible.
As you get fitter your body soon adapts and can provide the oxygen needed without your chest feeling like it's going to explode.
The solution is to run more slowly to give your body time to adapt and up its oxygen efficiency.
3. Brain saying "stop this right now!"
After years of inactivity running is a bit of a shock to the system. It's not comfortable and we all like a comfy life. The worse 1. and 2. above are the more your brain will be sending out messages like the red alert on Star Trek's Enterprise.
The solution is, of course, to slow down.
I think years of being told to run faster at school meant that it took me 9 runs to really get it and believe that slow running was nothing to be ashamed of - quite the reverse. I've never felt so fit in my life, or run so far, or enjoyed exercise so much - all due to slowing down.
Sorry for the long post.
Hope it's helpful if you're struggling with any of the above like I did.