Assuming you have some level of fitness in the past, you might consider doing the “1 bidon” routes from ZwiftInsider’s route badge list (Google “Zwift routes checklist” for a link, I’m still not cool enough to post links here).
ZwiftInsider also has a route badge plan but it kicks you off with a decent amount of riding off the bat, at least if you’ve spent quite a bit of time away from the bike and need to get your rear used to longer rides
Some of my favorite shorter routes from when I started are the Volcano routes, London classique, and basically anything that has no hills (Tick Tock, Volcano flat, some of the newer rides). You can sort the route lists by elevation ascending and go from there.
If you’re like me and are basically starting from scratch, the key is to take it slow and easy. Shoot for as many days as you can ride but start really small, like 20 minutes at a time. You’ll start to build up fitness quickly and pretty soon the seat pain will diminish or go away.
Also worth saying you should invest in a good saddle if you don’t have one already, and a bike fit if it’s been some time since you last rode. And good bike shorts and chamois cream - as a larger dude I’ve had good luck with The Black Bibs and Chamois Butt’r for longer rides.
And get a decent fan if you don’t already have one. Unless you’re taking a recovery ride, rides longer than 20 minutes are pretty awful without some serious air movement to cool you off. If you want to skip the middle steps look for the blower-style fans like the Headwind or something like the Lasko blower if you think that is overpriced (hint: it probably is).
Congrats on the new trainer and good luck!