Heavily Muscled Pony

[Kate Denisar <kdenisar@iss.net>]

I saw a posting that mentioned retraining a sprinter into an Endurance horse.  I have a half Arab/half Paso Fino 6 year old gelding.  He too is heavily muscled.  He has done very well at 25 miles, but we did our first 50 this spring and I had to pull him at 30 miles when he showed signs of tying up.
The causes of his ty-up were the result of an inexperienced owner (me) and a novice endurance rider (me again).  I was feeding him lots of grain (10%, 3% fat) with a topping of rice bran.  I was using cheap electrolytes rather than LyteNow or EnduraLytes.
After the panic subsided (mine), Spanky and I took a break for a couple of months.  I did tons of groundwork (TEAM) with him and really changed his attitude (he tended to be somewhat rebellious).  
I just joined a new company and moved to Atlanta, GA.  (I am bringing Spanky down next weekend - I can't wait! 8-)  I am concerned about a couple of things:
1)What preventive steps should I take to ensure that he will adapt to the climate, forage, etc? (He currently lives in the Shenedoah Valley of Virginia - near Harrisonburg.)
2)What kind of training regimen should I follow to help him compensate for his heavily muscled physique?
I would like to do a couple of 25s this fall and potentially move on to a 50 . . .
Kate Denisar and Spanky (the Mack truck at 14.2)