I had the same problem with my mare around 6 months ago. We moved to Denmark, where she was outside all day in the paddock and therefor eating grass all day long and then she was feed every morning and evening in the stable + the hay (wet hay, I don't know exactly the word in english). Way too much, so after 6 months of that I had to do something to make her lose weight.
In march I started with giving her just dry hay, as food just care4life (from Pavo) + 80gr every day of psyllium divided within morning and evening (coconut flavour and she loves it)which helps the "fat" to be distributed in all the body and not to be accumulate all in the belly area. She still go outside in the paddock every day (if it is not raining), I think it is good for her to be outside so she can walk, maybe put her in a smaller paddock, so she has less grass to eat, or reduce the hours she is outside.
Of course, she had to work more. As the others said before, trot until when you hear her breathing be more heavy and than let her walk. I would say at least 45 mins/1 hour every day and maybe you can combine this with some long outwalks? to make it more fun also for her :)
That's what I did and it worked but of course every horse it is different, it is just matter of finding what works for you two girls :)