Hi, I don't think there is anything wrong with your
bike or the gears. I too have a Giant Hybrid and it is a great bike for all round riding. Try just changing the gears one at a time to find the combination that will give you what you want. A heart rate monitor is an excellent way of
keeping your pulse rate where it should be. It sounds
like you're going into the anearobic phase if your
muscles are getting tired too soon. Take it easy and
try and keep your heart rate in the training zone. As
a general rule take 220 minus your age, and your heart
rate should be within 60-85% of that figure. To start
with use the lower end and as you get more fit work up
to the higher figures. Do this for at least 20 minutes,
to an hour and you should get a good aerobic workout.
Try for at least three times a week. Good luck!
Dave.