Turnout for Young Horses

[Jpcornes@aol.com]

I will be getting a colt at the end of this summer that I plan on using for
endurance some day.  I understand that in a perfect world he would be turned
out on pasture where he could run around, be a horse, and develop to his
fullest potential.  Reality is that I have approximately 1/4 acre fenced for
my 2 geldings (baby makes 3) with an additional 2 1/4 acre that is my
"backyard" which I can turn the guys out on (but not too often as in New
Mexico that equals making a dust bowl).

So my question is this -- given my limitations for turn out, is ponying an
option for developing muscles, bone, etc?  If so -- at what age and what
distance and speed?  I have access to everything from flat sandy ditchbanks to
mountain trails.   

Thanks!

Susan