You can go on a long ride on any kind of bike. You just count "long" in hours, not in miles. A 7 hour ride is a 7 hour ride, whether you go a century on a road bike, 70 miles on a hybrid, 50 miles on a mountain bike, or less on whatever your combination of bike & fitness make it.

There are a few reasons to get a different kind of bike. 1) You are riding with friends who have better bikes, and it is a struggle to keep up with them. 2) You want to bag a century. 3) Your bike isn't comfortable for a long ride.