A typical roundpen is 60' in diameter, so your 35x65 ft 'arena' is going to be half the size of that. Not enough room to do much of anything, and I've be very hesitant to work a horse at any real intensity with no relief from such tight turns.
I like Chasin Ponies idea of putting a large round pen in the middle of the pasture. It would almost create a 'paddock paradise' type track that might give the horse more movement in the limited amount of space.
“The horse is a mirror to your soul. Sometimes you might not like what you see. Sometimes you will.” - Buck Brannaman
"Nothing forced can ever be beautiful." - Xenophon