If she hasn't been in regular work, she'll need to be started slowly, just like we would; aside from muscle soreness, she would risk joint and soft tissue injury and exertion-related illnesses with a too fast start. Maybe start with some 10-15 minute sessions of walk-trot, doing circles and serpentines to keep her interested, stopping to walk her out when she seems to be getting breathy; alternate days with some hacking out in the open if that's available, up and down hills is good, but again pay attention to her fitness level and any signs she's giving that she's reached her limit for the ride. Up her times of exercise in weekly increments. Remember to warm her up (maybe a no-stress free longe) and cool her down, and stretching out her legs before working is a good idea. 250 lbs for her size is morbidly obese, which as the name suggests, increases her risk of the same serious problems a human would have, including heart attack, so go slow, but be relentless. After work a good bracer and/or liniment on muscles and legs will help reduce soreness and stiffness.
If there is a paddock/dry lot you could keep her in, maybe with a friend, so you can monitor her food until she has lost some weight and is looking more fit - that would be best. She'll look at you like you've betrayed her, but don't give in. Be sure she's getting free access to a good mineral supplement - I like the crumbles, the blocks are really not meant for horses since their tongues are very different from livestock and the blocks are too hard; the makers just throw them out there saying they are for horses b/c they're easy to handle. I use a small over the rail bucket at an appropriate height, sheltered if possible.She may start out gobbling them up, but as she gets what she needs, she'll slow down. When I started, my three would go through a scoop a day; after a month, they now use a scoop every five days to a week.
I am not one of those who in expressing opinions confines themselves to facts. - Mark Twain
The fact that man knows right from wrong proves his intellectual superiority to other creatures; but the fact that he can do wrong proves his moral inferiority to any creature that cannot. - Mark Twain