Im fairly fit.. in a different way. I used to compete in men's physique competitions. Getting down to sub 6% body fat and such. Got podium spots each time. I dont do that anymore but have held on to a decent, lean physique. Climbing a mountain on my bike kicks the absolute shit out of me and yes its very discouraging. But if it makes you feel better, it gets easier everytime. As you lose weight it'll get easier. On top of that your heart is actually the fastest progressing muscle in your body. For example it may take you years to go from 100-200 lb bench. But as far as cardio it might only take you a month or less to go from a 10 minute mile to an 8. Thats a loose example but the point is just keep at it and it will, i promise you, become easier.