Jenny, I know when you look in the mirror and your body seems so different, there's a huge part of you that is desperate to know that it won't always be that way. In 9 months your body changes in ways you can't even believe. And before the baby's here you can get very fixated on that. Firstly, when you have your little girl in your arms you'll be so full of joy and love that it will lessen they way you feel about how you look right now. You may feel very proud of your body and what it has achieved. You should do. It's very clever.
I'll be honest with you, immediately after you give birth, you will still look pregnant. Your uterus is still enlarged, your organs are squished and your body has stretched immeasurably. Don't worry too much in those first weeks. Breastfeeding will help your uterus contract back down and will use up extra calories. It will also allow you to sit and get to know your baby and recover.
In terms of exercise, well it's never a good idea to chuck yourself into a whole new regime if you're not used to it. Generally, a normal and active lifestyle and a good diet will see the excess weight go. Walking lots with the pram's good for you too in more ways than one.
It is entirely feasible, if not likely that you won't regain your normal shape. Not to say you won't be as attractive, as slim or the same weight again. Just that pregnancy changes you and that's just fine. I'm a completely different shape now- bigger breasts, slightly curvier but probably look better than I did before. I'm certainly fitter than I've ever been (I took up running when my dd was 18 months but just for fun).
The stretchmarks? About a year before they were silvery but they improve within a few months.
Please try not to worry.