First, some context - it takes a woman a lot of time and very particular effort to get 'abs like that.' I'm talking serious work-out time daily. I know - in my storied career - I have been boff. A woman does not get muscle definition easily - it's the way we are. Not saying Kate can't have abs like that - just that it takes a lot of effort and time.
I personally know women with similar body types that don't need to work as hard as other women to get well defined tone. Partly because they have so little body fat to begin with that any muscle tone they acheive is more pronounced and partly because they have been athletic their entire lives. Catherine "may" put in 1-2 hours a day, or she "may" put in 3 - 4 hours a week. Either way she can still be fit and toned. Considering her mother and sister's bodies I would say genetics play a huge role as well. The fact is we have no idea what her exercise routine is, only that she enjoys jogging and walking and we have seen pics of her and William coming out of a gym. Everything else is speculation.
So when it comes to that - what else does she have to do with herself? Recall that when William returned from the Falklands he announced that Kate would not be doing any more engagements until the Jublilee. The master had spoken (wonder how happy she was about that little bit of male dominance?) We all know that William 'has a job' and is 'very busy' - but what has he relegated his wife to? If we are to believe that they do not have servants, then Kate is 'busy' in her days squirreled away in Wales (where she knows no one) happily (we are to assume) cleaning the house, doing the laundry and cooking the meals for 'her man'. We all know how all that contributes to getting abs - not much.
Again, you are assuming she doesn't have anything else to do but work out, housework and cooking meals. You of course completely ignore that she went on a significant official tour of Canada and the US mere months after marrying, has had extensive prepping for her new role, has researched visited and established her patronship of various charities, made official appearances with William, Her Majesty, has had a significant role in the Jubilee and has done some less publicized visits as well. Not to mention, while she has been in London photographed shopping, she was also meeting with advisors, etc.. So really, the "down time" in Anglessy is very limited now versus what it was prior to becoming the Duchess and Cambridge.
And as it happens, Kate has far more time than even her mother has who runs a business. Poor Kate - I don't envy her - relegated to non-work (outside of housework) while her husband is 'working' and 'busy'. Kate's only prospect of 'work' is public engagements - for William to have shut that down for her - well, she has very little choice but to either watch television eating bon-bons or going out for runs and work-outs. More power to her - if that's her body, them's her abs - and she is doing great!
Yes, and her mother and sister who have careers also seem to have a lot of play time as well judging by all the photos of them doing just that. It is possible to do both. Kate's "job" is the Duchess of Cambridge. She cannot get a job outside of the BRF, and while William is stationed in Anglessy with the RAF, she needs to be there as well in order to spend time with her husband. Even then, neither one of them can be entirely free from their duties considering the advisors, personal secretaries and staff they have in London who need to be in contact with them even in Anglessy. I seriously doubt she has much time to sit and watch the telly or working out all day. Her leisure life was over the moment she said "I do" to the second in line to the thone, and the fact is we have no idea what her typical day is like in that role.
As to William "shutting it down for her" and forcing her to be just a housewife, that is truly ridiculous. I have had the impression judging by the Jubilee that William's time in the RAF is coming to a close. She has IMO been doing a fabulous job stepping into her role and she has certainly exceeded the BRF's and many naysayers expectations, however if William's time in the RAF and Anglessy is coming to a close they both have every right to enjoy what time they have left there.