Starting endurance

[PSCHOPony@aol.com]

I want to start my 4 yr old QH on endurance.  He is in fairly good shape, but
we never did any series miles yet.  What do I feed my horse and how much?  How
do I measure distance, such as miles, since I would be working on trails?
What about hill work?  How do I know when my horse is being asked enough or
too much?  What should I do about cross training, e.g. barrel racing, jumping.
Thank you.
PSCHO Pony and Spirit