I use a simple Speed-Distance-Time calculator (
https://www.calculatorsoup.com/calcu...calculator.php) and input the avg. speed I think I'l be doing that day, based on whether it'll be the road bike, commuter, mtb, and if I'll be hammering, or doing a slow recovery ride, and how much climbing will be occurring, as well as wind direction if it's just a point-to-point ride, so anywhere from 11 mph to 21 mph, which changes ride duration dramatically.
If I know I only have 2.5 hours to ride, I can use the calculator to tell me how many miles I can fit in, by inputting that time and my avg speed, and then think about a route of that length.