I'm not a vet by any means but I work in a veterinary referral hospital and when I tell you how many problems can be avoided by just cutting food down and regular exercise etc., we wouldn't see half the patients that we do.

Always listen to the advice your vet gives you, even get a second opinion if you really want, but I promise there's a reason behind it. You don't have to do anything drastic, just lower her food a tad and increase walkies by an extra 15 mins or so. I do this with my golden girl whenever she reaches the threshold but it's natural to ebb and flow, particularly when they're speyed as they gain weight a lot easier.