Thank you John. I really love your program, btw! It's amazingly helpful to figure out the best path so I don't end up in the middle of the desert in August!
There isn't a way to add multiple markers automatically, but you can add them manaully in the Directions panel of the route planner. You'll be able to see the distance from the start as you add each one.
I'm walking from Charleston, SC to Newport Beach, CA. I've created my route. I'd like to drop a marker every 10 miles along the route to gauge where I'll be landing each night. I'll walk anywhere from 15-25 miles/day and would like easy markers to see every 10 miles along the route so I can search nearby ahead of time for camping spots.