Cycling wears you out and because surfaces aren't always the same, you can end up needing more energy at some times and less at others. When you speed up and down those hills, your heart, lungs and muscles soon get tired! So if you're new to cycling, you need to pace yourself.
Don't set off on a marathon bike ride if you're not used to prolonged exercise and you haven't done a bit of training first! Don't try and do too much at first; build up the distance and intensity of your rides as you improve, unless you want to suffer!
Stay within your limits, especially during the first few hours when you're feeling full of energy - it's easy to tire yourself out too soon. If you want to go the distance, pace yourself.
Don't stop too often and don't stop for too long so that you don't let your blood pressure drop - it only makes it harder to get up and carry on again!