How to build up fitness when pregnant

Before becoming pregnant I was pretty unfit, I used to walk 2.4 miles a day but haven't done that in a year now, now I rarely leave the house, and I get out of breath walking up stairs / up a small hill. I want to be able to do a 7 km / 4.4 mile walk when I'm 35 weeks pregnant, I'm currently 18 weeks. How should I go about building up to that without tiring myself out too much / hurting baby?