While I know you aren't ready to buy a horse yet, take this advice for what is it worth and in the well-meaning spirit to which it is intended. Whether it is buying horse or deciding what path to take in life, if you have to "squint your eyes" to make something look right for you then you need to take a better look with eyes wide open.
What that means is that if you have to overlook your main goal (squinting past it so to speak) by saying "it doesn't matter I'll just make it work" ...well it probably won't work out. In this case the owner flatly states the horse would rather go slow than do competitive long distance, which makes her unsuitable for your main purpose. When you try and make a horse do something he/she is just not cut out to do both you and the horse end up frustrated. Better to look around more for a horse that wants to do, and is able to do what you ask.
Just my 2 cents.