Im purchasing a new bike today, it's a Mongoose Maurice FS 2012.
What's the practicality of having a single speed hybrid for long journeys like?
I have a road bike already I just thought a single speed hybrid would be great for fitness and some of the trail/off road detours I used to take on my mountain bike.
Practicality is going to be defined by you, in large part. You'll have to consider if there will be any hills that you'll have to ride and if you'll be carrying in significant loads (aside from your corpus) up hills. You just have to be willing and comfortable with having only one gear.
Some folks enjoy the simplicity of single speed bikes. Maintenance is simplified a bit. Chain life of a single speed is typically longer than that of a normal geared bike (i.e., one with a cassette and multiple chainrings up front) since the chain doesn't move between gears and always has the optimal chainline.