At the beginning of my triathlon plans I was confused about the terminology as well. You should read this SUF articles about running. Especially „understanding running workouts“. Everything will be explained.
If any questions remain, feel free to ask. 
The need for specific duathlon plans was already discussed right in this thread. Since I already stated my opinion there and am too lazy to type up or copy another reply, forgive me if I just link to it.
My quick idea: Have a read through the linked article. Do a triathlon prep week to figure out your running „numbers“. Do a 12 week tri plan and sub swimming for strength. If you like it, do a custom plan or continue this road until proper duathlon arrives.
I always ran with a gamin watch and preloaded the workouts onto the device. It took some time creating the workouts initially, but after the first plan only short modifications to the existing data sets were needed. Worked like a charm.