If the saddle you pulled off your MTB is working, then it's working! What's the longest ride you've done with it on the road bike so far?

My experience is limited to my hybrid, but I'd agree with the others that a different riding position might mean different saddle needs. If you can go two to three hours with the same saddle on your road bike and no problems, I'd say you're good for the long haul. If you start to get any chafing or pressure at all at that distance, I'd keep looking.