First, I plot the route in RWGPS. I notice the estimated riding time. I then look back at previous rides in TrainingPeaks, looking for a similar ride in terms of feet per mile and then notice its TSS/hr. I multiply that times the estimated time for the new ride. When I create the new ride in TP, I click on Build Workout, choose Threshold HR or FTP, Duration, and Target. Then I drag down an Active block and then enter the estimated elapsed time, guess at an effort number, and then manipulate that until I see the desired TSS. I add a ride title and save it. If it's a ride I'll do again, I often drag the completed proposed ride or workout over into my Library of workouts.
I do this with all my workouts, a couple weeks in advance, so I can plan my CTL and TSB progression. Of course things never work exactly as planned, so I edit, add, or subtract as I move through the week(s).
The above assumes that you'll ride the new route with the same intensity you used on the route from which you got your TSS. You'll see the power or HR average necessary to do that when you create the new ride or workout. Mess with that number if your want to change the intensity for the proposed ride or workout, then try to ride to that intensity.
Takes practice, but experience begins when you start.
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Results matter