Congrats on having a baby girl! The age six months is so fun in some ways because it's the time when babies start to get more interactive, but it's also so hard since their needs are so high.
I'm a huge advocate of walking as the best form of exercise when you have a little one. There are many different types of baby carriers and strollers to choose from, so you can probably find something to match your lifestyle and price range. Walking is nice because it's low impact, low cost, you can bring your baby with you, and you can always stop for feedings or diaper changes!
When my daughter was tiny I absolutely loved my CuddlyWrap, a stretchy wrap carrier. I could wear her comfortably in it for hours at a time and one time I hiked in the mountains while she slept for five hours straight! There are many different types of baby carriers and if you have any sewing ability some of them can be easy to make. (I can't sew to save my life so I'm going based on what my friends tell me!
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In some areas you can find mom and baby fitness classes (strollercise, mom and baby yoga, etc.) at recreation centers or private facilities.
Remember to be gentle with your body. It took almost a year to put on the pregnancy weight and taking at least that long to get it off again is very normal.
(And as a total aside, I was an unexpected mom at a young age too. It can be so tough at first , especially when you don't have a network of mommy friends. Big hugs to you!!)