Welcome to SA.
I live in Tokai and the trails here are really nice - you may find a few sections after recent heavy rains where you may have to carry your bike for a short distance to clear rockslides, etc.
I can just start by mentioning that all trails and routes are mostly safe over weekends with influx of walkers, cyclists, etc. You are more likely to run into trouble 10h00 on a Tuesday morning when the mountains are quiet.
From the Waterfront you can do 2 routes to Tokai area.
1 - You can ride along the road that runs all along the beach through Camps Bay to Hout Bay.
From there you can get into Constantia side via Constantia Neck, where you can access local Green belts and make your way to Tokai trails. Riding in Tokai you can go up to the Tokai Mast (+-800m elevation from the gate), ride into Silvermine and do a loop around there, you can go down what is called the Donkey Trail back into Tokai and ride all the fun trails back down to the gate.
Tokai is not very well sign posted if you are new to the area, but people are very friendly and always willing to give you advice and directions on which routes to take to the most fun parts of the trails.
2 - You can cycle from the Waterfront to Table Mountain and cycle to the Kings Blockhouse.
You will find many cyclists on this route most late afternoons and plenty on weekends.
You can go back down the mountain into Newlands Forrest where you can cycle through the trails over Kirstenbosch Botanical Gardens which ends/starts at Constantia Neck.
Same as above, you can access local greenbelts, etc. from here to Tokai trails.
Both of these routes will give you 100km easy with something like 2000m elevation. Possibly a lot more if you decide to go to the top of Tokai Mast.
Finally, I would stay off the cycle path that runs from the city to Milnerton/Blouberg beach.
However, from these areas you would also be very close to the Tygerberg Trail Network.
