There are a couple possible options:
I am attaching a small model that I created where the operator travels exactly three seconds from the pickup queue to the dropoff queue. The operator then travels back to the pickup queue in exactly six seconds. This is accomplished by setting the operator speed to 1000 fps and the acc/dec rates to 0 (instantaneous acc/dec). The speed limit for the connection from NN1 to NN2 is set to 10 fps and the current distance is 30 so it takes exactly 3 seconds to travel from NN1 to NN2. The speed limit for the connection from NN2 to NN1 is set to 5 fps and the current distance is 30 so it takes exactly 6 seconds to travel from NN2 to NN1.
A virtual distance will also work, which is probably the best way to go in case nodes get accidently moved around. If you want to make the travel times variables you could create a global table with all of the different path segments listed in the table that you want to make variables. All of the virtual distances could be modified in this table and reset before running the model in order to modify the travel times between nodes. I am attaching another version of the sample model with these modifications included. The reset trigger on each node is where I set the virtual distance for each path segment. I also added macros for referencing each path segment.