Well, they were running from the explosion of the Omega 13... I mean Genesis.
But since the Genesis device wasn't the only thing that could explode on the Reliant, they were also running from everything else that would be ignited - mainly her anti-matter tanks.
But that's my point. Do we know what aspect they feared would lead to their destruction? We know that the Genesis torpedo is meant to be fired at a planet and detonate and do its magic on impact. How do we know how destructive its wave would be at a distance away from the original detonation? My point was perhaps that they were more concerned about the explosive force of the warp core breach than the Genesis wave.