Stop worrying about all that now n take care of ur self n baby.. u can do; that after ur body heals completely. That is mostly after six months Just enjoy ur mother hood now.. all the best

As it took 9 months for your to gain weight it will take that much time to get back to shape, After you complete 3 months you can start with little walking.. Check with your gynaecologist when you can join gym.. usually it should be done after 6 months.