pony with heart murmer

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

Linda Dewees quiltrider@hotmail.com
I am a new beginner at endurance and would like to try some LD rides that are in my area this winter.  I have a horse and a pony I'm riding now.  Probably neither of them are perfect for endurance, but at least they'll get me started.  My horse Beau is 15 and very lazy, but I'm hoping he'll perk up as he gets in shape.  Rosie is a 12 year old POA mare.  She's 13.2 hands and I'm 5'4" and 120 lbs.  Is she too small??  She is energetic, but takes care of herself, and seems to carry me fine.  I was also told by a vet several years ago that she has a prominant heart murmer, but since she's still alive and kicking it must not hinder her much.  She seems as normal as any of my other horses.  I'll get this confirmed by another vet, but if she does have a murmer could she still do LD rides as long as it wasn't affecting her heart rate?  I've taken her HR after exercise and it's always been below 80 bpm.  It drops fast while I'm getting off, getting out the stethescope and finding the right place!  
Thanks for all your knowledge.  I've learned sooo much over the past 6 months reading ridecamp!