Conditioning young/beginning horse

[Kirsten Price <kirsten@vermilion.reno.nv.us>]

I am just starting out and have a couple of questions.  I have a 9
year-old Morgan gelding who is currently in excellent condition.  He's
actually a cutting horse, but I'd like to take him on some shorter
endurance rides throughout the year for a change of pace so to speak.

But my question is really about my 4 year-old Morgan filly.  I'd like to
get her started on Endurance/CTRs.  She has about three months of
training with an excellent reining/working cowhorse trainer under her
belt and she is solid on the trail (even some rough stuff) over short
distances as the trainer uses hill work and flat work two days per week
as part of both mental and physical conditioning to supplement the arena
work.  I'd like to get her in good 25-mile condition by mid-spring, but
am concerned about doing too much with her.  She is mentally mature but
physically immature as yet.

I searched the archives and got some good info... but no one ever said
anything (that I could find) about ponying and how effective or
ineffective a method that would be to condition a young horse without so
much stress from a rider. Also, is it typically acceptable to pony
well-behaved young horses on rides?

I do not have a heart-rate monitor.  How important is it that I get one
(also how soon) and what brands are recommended?

I'd appreciate any information and advice anyone has to offer.  Thanks.

Kirsten