If you are set on riding bareback because of the freedom of movement, you could compromise with well made treeless saddle with a treeless saddle pad that distributes the weight better for the horse. (Your butt will thank you, too!).
Conditioning usually begins with long, slow distance and then involves fartlek training through whatever terrain you have access to. It is good to ride through as varied as you can, but it's not always possible. Where I live there are no hills, so I find deep sand on rides and we canter through it to build wind and muscle. Arena work is good for balance, control and communication as well as some light conditioning.
Endurance 101 is a book on Kindle and it will help with the basics. A good, light read.
As for riding professionally for money in Endurance, I don't know anyone that does, so I cannot help you there. Let me know if you figure it out - It would be nice as rides can be expensive!