Spyder: (and correct me if I'm wrong) But I believe your point is that she won't be able to be competitive in both simultaneously?? I would definitely agree with that as they require very different focuses.
OP: I think that doing some simple, basic dressage work will be great for you. Learning how to properly work with your horse can be a valuable tool on the trail. Obviously you want to keep your focus on the conditioning, but if you have the time/commitment to do some dressage lessons as well, then go for it! I'd make sure you do a lot of good conditioning (preferably with an experienced endurance rider helping you) and some more 25/30 mile rides, and then probably a few 50/60s before you do anything longer...again, having someone with more experience to help you would be a HUGE benefit so that you can do this safely and make sure your horse is truly fit for the distance you're asking of him/her. You really don't want to try 75 on a horse that isn't ready. It could be very dangerous.
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.