How do you know it is too hard to get into? Hundreds of us get in every year. If you know that you want both an MD and a PhD, I don't see why you would automatically write off a combined program a "too hard to get into" without even giving it a try.
On the other hand, if your grades/MCAT are too low to be competitive, science graduate programs are much easier to get into than medical school, and having a PhD would likely give you an advantage in med school admissions. But I would encourage you to not go that route without trying the MD/PhD admissions process at least once.