As it is April of your junior year, to have an Ivy athletic recruiting commitment at this early stage implies that you are pretty good at your sport. It is my experience that these sort of commitments, while not binding, are taken pretty seriously by Ivy coaches, so by committing to you, there are other athletic recruiting hopefuls that will have to look elsewhere. There is a domino effect to these decisions.
It seems to me if you are not sure about fully committing to this school, you are being rather selfish to hold a place while you keep looking. It is widely known that Ivy League schools do no give any merit aid so if that is a factor you should be honest about it. I also think the world of athletic recruiting (depending on sport of course) is rather small, so if you are still looking after committing, the coaches will know/talk.
As a separate matter, Ivy/D1 sports can be very time consuming, and premed tracks at most Ivy’s is very intense. While there are those who have done it, it is very hard. Given the importance of grades/scores to get into med school, I often think going to an in-state option can not only be less expensive, but also academically less intense, allowing for more time to enjoy the college experience while still achieving a med school goal.