Others with more experience maybe able to give a better answer, but I think there are a few answers that can be given.
I've been a cyclist since the age of nine and an athlete throughout high school and college and from what I've experienced and seen is that each person has a different metabolism; mine is very fast so I can eat like a horse and not gain much...some people have fast metabolisms, but there getting older so it has slowed down a bit. Then you have those who have slow metabolisms period. If your metabolism is slow it becomes easier for you to gain weight and exercise/diet becomes more important to you than for others. I agree with Princessa about
the rice and pasta, but I wonder if you can reduce the portions of rice and/or pasta and maybe increase veggies or some of the other food items mentioned by Princessa? Another good reason to look into Princessa's suggestion is constipation, starches have a tendency to cause stool to bind together, which is why vegetables are so important to keep the bowels moving.
It's a shame you can't 'magically' transfer some of that weight to me…lol
Since I stopped eating starches a couple of months ago, I've lost a lot of weight, but I only have two more weeks to go on this Candida elimination diet I'm on. One last thing about
exercise, even if you're very slim, it's always good to exercise; when you exercise you are actually oxygenating your body, which kills bacteria; it expands the lungs, strengthens the heart and clears the arteries…but the key in my humble opinion starts with diet.
Too many fruits taken at once may cause a problem for UC since some of them are high in sugar even though it's not processed sugar.
Good luck and I'm sure you can and will lose the weight you desire,