Usually I start by estimating the average power I want to ride at (e.g. Z2 endurance, Z3 tempo, etc). That'll give me a rough/estimate IF.

Then based on the effort, I make a rough guess on the ride duration. Often this will be in terms of average speed (e.g. flat route, avg 30km/h, hilly route, 25km/h, etc), so distance / speed gives duration. With IF and duration, I can roughly estimate TSS as IF * IF / duration (yes, I know the actual formula involves NP/FTP but IF * IF, for me, is close enough, for "rough estimate" purposes, and also easier to do it in my head).