Really tired, pregnant with twins?

So as expected I'm exhausted. I'm about 13 weeks, pregnant with twins, believe me I'm so excited but when does this extreme exhaustion end. I know some women get bursts of energy at some point in the pregnancy but is there anything that I can do so I can at least make it to the gym? I'm trying to stay in good shape through out my pregnancy so I can get back into shape after I have my babies. I don't want to take any pills. I already take my prenatal vitamins and everything, I get enough sleep, my husbands sort of worried, even though I know this is normal. Any advice would be nice.