it's probably a good idea for you to drop by the doctor's office and discuss this with him/her. it's not going to hurt anything to talk to the dr.
a couple of things here...first, it's not enough to sip water. you need to drink water...as in take a 16.9 oz (common size) water bottle with you and empty it while you're working out. dehydration is a major problem and when you're exercising, you need to be especially hydrated.
second. breathe. people in gyms forget to breathe all the time!!!!!!!!!!!!!!!! you watch...you'll see people all red in the face in gyms, because they're holding their breath through exercise. this includes ppl who are walking. get into a regular pattern of deep breathing while you walk. breathe freely.
also when you're walking, you should never be walking so fast that you cannot hold a regular conversation. this is a good indicator of whether you are putting forth too much effort. if you find that you're too winded to carry on a conversation, then slow it down a notch until you feel more comfortable.
remember, when you go to a gym and you haven't been to one in a while...your objective is to get healthier. this doesn't occur all at once...you're not punishing yourself for eating too much or for not exercising...you're just trying to get healthier.
start small and work up to it. your stamina WILL improve if you just stay with it. if you burn out early, you'll never be able to keep up.
there's a joke out there on the internet currently that says something like "i made a new years resolution to lose weight and work out at the gym...today i put on some sweats and got in my car and rode past the gym...i feel better already!".
well...there's a point to this. start real slow and work up. don't be afraid to step in the baby pool (so to speak) walk the treadmill for 10 mins and see how that goes. if you feel fine afterwards, then go for 12 mins. the next time. if you feel fine then go for 15 mins the next time. if you start to feel dizzy, then back off to 12 again for a few times until you feel the energy to do 15 again.
don't push yourself too hard...remember this is to improve your overall health...not to punish you. it's a gradual process and if you are smart about how you do it, you'll gradually lose all the weight you want or you'll develop the stamina you want, or you'll achieve the fitness level you want.
good luck! you can do it!
jasmine