Feed/conditioningprogram for easy keeper?

["guest@endurance.net" <guest@endurance.net>]

Sally Hafkemeyer hafke7@netins.net
I'd sure like some feedback <g> on my
feeding/riding program for my easy keeper.
I'm from NE Iowa, ride in the Midwest region.
I have an 8yr gelding,  1/2 Arab, 1/2 Qtr. 
He's 15.1, weighs 1000 -1100lbs. 
Big-boned, size 2 foot. Resting pulse 36.

He is kept on 10 acres pasture, ABC
Plus, and this year I added ABC Free Choice
minerals and Dr. Beechers E-Sel supplement.
(In winter I feed all the grass hay my herd
of 5 will clean up.  Mostly first cutting,
lots of fill, not much nutrition.)

 I did a couple 25's last year with him,
he had very good recoveries, and seemed to
enjoy himself.  This year I started him on 50's.
Began conditioning in January, slow to
moderate work in snow and hills.  We completed
four 50's so far this year.  Avg ride time of
7.5 hrs.  At that pace, we trot in to VC and
walk right up for PR.  Pulses in below 60. 
My goal has been to build his base, mileage
with no negative effects, future as 100 miler
horse.

Ti, his usual at-home working heart rate is
80 bpm.  I've gotten him up to 120 on 1/2
mile climb of 6% grade.  I also use a 2 mile long 
uphill grade for regular conditioning,  steeper
as it gets to top.  I've never had his pulse
higher than low 80's on this stretch with aprx
7 mph working trot.  I've been reluctant to
push him much for speed, as I've wanted to
put a good base on him.  Thought my HRM
was malfunctioning, but jumping off and taking
pulse confirmed the HRM reading.

This is a laid-back boy, very controllable at
start and on trail, good hauler and camper...
just a real pleasure to be around. 
  
My concern is his energy level.  He gets 1 cup of
grain, just enough to put his supplements
in.  He looks a little heavy, even considering
his qtr horse genes, eats constantly on trail.
My first ride he actually put his head down to
eat grass while all the 25 milers blew by us.  
He goes willingly, but acts as if he's completely
starved from start to finish.  The last two rides
he had all A's at all checks except for gut
sounds of B.  Vets tell me to be sure to let him
eat (they just don't know!)  He scarfs beet pulp, 
mixed with 2 cups sweet feed, apples, etc
at all vet checks, besides all the grass on
trail.  
Is this just a lazy horse, or could I be doing
more to increase his energy level.  I have 
NEVER had a horse like this, so feel in
unfamiliar territory. 
Any thoughts or ideas would be greatly
appreciated.  Sally H.