The FoCo Fondo training rides are great training routes for SBT GRVL and will give riders exposure to different terrain (and you might also want to consider their awesome event if you aren't already signed up!)
Get the info for this event straight from the source: https://www.focofondo.com/training-rides-events/june-training-ride-2023
NOTE: The July ride has moved to a virtual format because it’s FoCo Fondo month. The July training ride has two very adventurous options, centered around the Pennock Pass. Both routes are pretty big with the amount of climbing and altitude and are much more fun with friends.