I really love mountain bikes, how about you?
I have a friend that does it - they are a bit different from regular bikes as they are enhanced for durability and performance in rough terrain.
That is true, they have special wheels that allow the bikes to perform better on and off trail.  Biking on the beach is off trail too, and that is hard!
She rides hers mostly on mountain trails, fire roads and other unpaved surfaces.
That's great.  It's great exercise.
I didn't realize they were popular with couriers who have to navigate potholes and over curbs also.
I did not either!  Come to think of it, I have seen couriers with some sturdy bikes.  They are mountain bikes for sure.  I actually had a ten speed and was hoping to have a tire that was more like a mountain bike - because of the pot holes!  I could only get normal tires for it.
rims and wider tires has also made this style of bicycle popular with urban riders and couriers who must navigate through potholes and over curbs.
Since the development of the sport in the 1970s, many new subtypes of mountain biking have developed, such as cross-country (XC), all-day endurance, freeride, downhill, and a variety of track and slalom types. Each of these place different demands on the bike, requiring different designs for optimal performance. MTB development has led to an increase in suspension travel
Off-road duathlon
Off-road duathlon is a form of duathlon, where the competitors have to go through a trail-running stage and a mountain-biking stage.
Off-road duathlons are distinguished from conventional duathlons in that the terrain for the cycling and running stages are generally unpaved, rough, and very steep and hilly. They require different techniques than conventional duathlon races, and the athletes employ mountain bikes rather than road bikes.