Ridecamp: [RC] RC: First 50?

I'm trying to decide whether to do two 25's or a 50 at Cascade Challenge next 
weekend.  How many miles per week were you riding when you did your first 50? 
 My mare has done a total of 150 LD miles, 100 last year and 50 this year, at 
an increasingly faster pace.  She seems to do well, and has a lot of energy 
left.  The comment I hear most often after a ride, as she is dancing around 
because her buddy has left or just standing by the trailer, is that I should 
be doing 50's with her, I didn't get her tired enough.  I have about 400 
miles in LD, the rest on her mom, who did not seem to have the 50 mile 
potential--ran like he!! then real tired after--no matter what I did.  

I THINK I might be ready for a slow 50, but having never done more that 25 
miles either in a ride or conditioning, I'm not sure if I should push my mare 
that far.  I have been riding about 20-25 miles a week in conditioning and 
taking dressage lesson once a week.  I'm also afraid that the more 25's I do, 
the harder it will be for the mare to jump to 50's.  Opinions, comments, 
conditioning regimens? All will be appreciated.   jeri

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
 Ridecamp is a service of Endurance Net, http://www.endurance.net.
 Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=