Where are you based? If you in Durban itself, there are no trails in an around Durban. You have to travel a bit. Closest to Durban is the Stainbank Nature Reserve, where you can do a few laps on relatively easy trails to get in a 30km ride. Expect some relatively tough, short climbs. It's in Yellowood Park, about 20 minutes from CBD.
North of Durban you will find the Sugar cane Trails that are very popular, and many groups ride them on weekends. Best to do them in a group. There is a ride from Umhlanga to the Airport and back that is all off road in the sugar cane fields. Pretty boring and technically not very challenging, but if you dislike gnarly, technical terrain, this is ideal.
Further north, you have Holla Trails in Ballito, about 40 minutes outside Durban. Holla has miles and miles of really good trails. Suitable for beginner/intermediate and very safe. Holla has a few sections you could regard as technical, but nothing that requires high level skill or 160mms of suspension travel. Highly recommended if you want mellow sugar cane roads and manicured, flowing smooth singletrack.
South of Durban, you have the Rocky Bay trails near Pennington, about 45 minutes from Durban. Very mellow, easy trails that start from a very picturesque caravan park. Nice place to spend a day, ride in the morning, and have lunch in one of the seaside towns in the area, or go play golf at Umdoni Park, one the prettiest golf courses in South Africa. There's also a few excellent surf spots in the area if you like catching a few waves, as many Aussies do.
Inland from Durban, you have the trails around Shongweni, which are very busy on weekends. You can make a 60 to 70km route here taking in some road, jeep track/sugar cane and easy singletrack. It is best to find a group to ride with as there are some dodgy places where people have been mugged.
Close to Shongweni is Giba Gorge in Pinetown. People sometimes ride into Giba Gorge from Shongweni and it is an awesome ride. You can drive to Giba and park in a safe, secure parking and ride the trails. But, you won't get a long ride, unless you ride out of Giba and back in. Giba does have some easy trails, but it does require some skill to ride all the trails there. There are lots of technical features, it can be steep and you if you stray onto the enduro trails, you will quickly find that Giba is a proper test of your skill and your bike. It is a lot of fun if you want to be challenged. My favourite place to ride within 30 minutes of Durban.
Further inland you have Virginia trails (about 60 mins from Durban) which has excellent beginner/intermediate trails. And, just under 90 minutes from Durban, you have one of, if not the, best trail networks in the country, Karkloof and Howick. Miles and miles of excellent trails of all kinds. You'll find everything from DH to mellow gravel riding. Karkloof is a joy to ride and you will definitely find suitable trails. The locals are friendly and will point you in the right direction. And, it is safe, as long as you steer clear when they felling trees. They usually close trails when there is felling.
Wherever you choose to ride, try chat to the local riders about safety issues, and when there are group rides. KZN people are generally friendly and will be helpful.
Good luck finding somewhere to ride. I would suggest going to Giba and meeting the locals there. Speak to Karen and Fiona who run coaching clinics there, they will happily show you around and they know all the best places to ride. And, you'll get some great tips on improving your skills.
