Cessna if you can get rat blocks instead of seeds and fruits. Purinaused to make rat blocks, they are great for their teeth and it’s very rare for a rat to get overweight using them, I’m sure there are other manufacturers but I have not need them for about 5 years. I bought 20 pound bags from the feed store, divided them to half pound bags, used vacuum deal and stored in the freezer with the other critter foods to ensure they stay at their freshest.
In the mean time get your rate to exercise, swimming is excellwnt for them, my son uses our tub and puts room temp water in the tub about four inches deep, he has a big rock island his rats can climb up on when they have had enough and they really enjoy their swim time, they don’t try and jump out or panic. He starts them out by putting them on the rock and let them take to the water themselves. Since your rat hasn’t swam before you might lower her don on your hand so she doesn’t panic. Most rats love swimming, they are quite natural swimmers and if the waters not too cold or hot they do keep jumping back in just for the fun of swimming.
Other things are rat balls if you can find one big enough, but you have to clean them every time they are used or they get gross.
Cut back on seeds for the diet, more fresh vegetables like carrots etc, even dried vegetables they seem to love, very little fruits like 1/4 banana chip or 1/8 apple slice dried.
Exercise your rat as much as possible, like us they need to build their stamina you can’t put expect to start with 30 minutes and hope the rates not tuckered, a good 5 minutes twice a day adding about 3 minutes every other day because she’s mature and over weight, let her acclimate to the new routine. Also just carrying her around on your shoulder or in a pocket she’s not sleeping concerting energy and calories, it’s better than curled in a cage sleeping. Ad small boxes like single cereal boxes or pop tart boxes - similar sized ones. I use a pet safe glue and make box condos with holes between the boxes too smell for the rat to get into to explore so they spend hours chewing their way into the different compartments eventually eating the majority of the boxes (they chew and spit out all the cardboard and glue and nest in it). Cheep fun, entertainment and replaceable when they get smelly.
Another thing you can do is leash train them, make a small harness out of parachute coats or similar, use a jewelry clip to open and close it and the same cordage to keep them in range, its basically a band around the neck, and down the back with a band around their front arms, then connect the belly band underneath back to the neck. We didn’t have to make the collar adjustable but the belly band was needed to open and close. Very similar to dog harnesses, snug enough around the body just behind the front legs and the collars looser. My son keeps his guys leashed and then takes them outside to explore in the garden. They do come to a clicker but he keeps them on a leash just in case if neighbors cat or the temptation to scurry up the drain pipe, he does not want to take the chance they run, but wants them to have the freedom to explore. His rats live. Around 5 to 6 years depending on the breed.