The idea that a whippet needs a lot of walking is a bit of a misnomer. They don;t need hours and hours of walking because they are built for short sudden bursts of energy, so they tend to need several shorter walks rather than one long one.
Anything that is normally bred as a working (farm or field/gun) dog will need lots and lots of exercise so labrador, retriever, labradoodle, etc will go loopy and can be destructive if they aren't walked.
The thing with walking dogs is not necessarily how far/long they need, it's about the mental stimulation it gives them. They need to be able to follow smells and use all their dog instincts in order to tire themselves out mentally as well, it's not simply about running of physical energy. Are dogs allowed off lead in the park? If they aren't you need to double the time they need walking for as being on lead just doesn't give them the same quality of walk as running free off lead.
Given that you have a very young child now and the age gaps you foresee for your next child/ren you will be walking the dog accompanied by babies/small children for at least 6 years. How will you walk it when you are heavily pregnant/have a newborn (plus toddler/s)? Consider the implications of dressing 3 children for the weather, getting them out, manhandling a buggy and puppy/dog plus 1 or 2 more walking children at least once a day, ideally more, no matter what the weather. In the winter when your dp/h will be leaving for work in the dark and coming home in the dark, how will you manage to fit in walks? What if the dcs are ill? My 3 dcs can be very reluctant to come with me on walks and it is not a lot of fun dragging 3 whinging children out in the rain
.
We got our dog (lab) when our youngest was 20mo and the older 2 were 3 and almost 5yo. With hindsight we should have waited another year at least, if I'm honest. The puppy stage, especially from 5mo - over 1yo is very, very hard. It is as bad as having another baby/toddler, if not worse. We lost so many toys because the dcs weren't old enough to understand that whatever they left around would be chewed.
They need so much input and training every single day, on top of the walks which I didn;t take into account. Fitting in all the dcs' needs while socialising, training and walking the puppy was almost impossible some days. Training was also trickier because the dcs were too young to be part of it and loved to career around with her and wind her up but then got upset when she nipped/scratched them. In effect I had to train all 4 of them.
I have to say, there were many, many times when I nearly gave her back to the breeder because the stress and amount of work involved was so huge.
She's almost 3 now and has settled down into a wonderful pet but I wish we had waited because it would have been so much easier for everyone.
Personally, in your circumstances I would wait. We were swept away by the lovely adult family dog idea, we had no real idea of how much work it would take to get to that point, despite both of us having grown up around dogs. Have your children, let them grow up a bit - dare I suggest, start school - THEN get a puppy.