Best Exercises For Mesomorphs ?

I find I gain muscle easily, especially in my calves and thighs, and would really prefer to be more of a streamlined shape.
I seem to have got stuck in a rut of just cycling most days as it does use up a lot of energy, given I have to go up and down quite a few hills and bridges etc. But I want to mix it up a bit and find some other exercises that might balance things out as cycling tends to be only using my leg and buttock muscles and my calves and just above my knees are becoming muscly.

I used to swim and go for 2 hour walks, and have been debating joining a local gym with swimming pool but am worried that other exercises wont be as efficient at burning off calories ( I am guessing that 2 hours cycling including hills, must use around 1000 calories).

I don't seem to be losing any weight or even really managing to maintain my weight that well anymore with the cycling, so am wondering what would be the best exercise for me to do. I cannot really run as I have mild COPD and would get too breathless, I think.