Since you are just starting out, like myself, I find it best not to alternate between very fast, and then going very slow, as a prime result of just pushing yourself too hard. Not sure if you are doing that, but what I suggest you do is keep your rides at a steady pace... One that you are comfortable riding at. This way, your body gets into the rhythm and uses up less energy overall. Another thing: Filling your diet with complex carbohydrates aids in higher energy levels throughout your rides.
And last but not least: HYDRATE DURING YOUR RIDES!