My only concern is she is not fully grown at 4 or even 5yrs old. So going for conditioning miles at this age could be a problem in the long run. Bone does not finish growing, even in smaller animals until at least 6yrs. There are no “early maturing” breeds. Some well fed animals get bigger at younger ages. This does not mean they are mature, with bones hardened like adult horses. Bigger animals than OPs take even longer to finish bone growth.
I have no concerns with you riding her now for shorter miles, or when she gets mature, on competitive rides. I just think 25 mile rides at 4-5yrs, is asking too much when added to conditioning work she needs to be fit enough.
A suggestion is to pony her from your other horse, letting her see things, get gentle miles with no extra weight on her. I would still keep her mileage reduced from what you ask of a mature horse. I have found ponying youngsters to be a huge aid in mental development, helpful in SLOWLY INCREASING her bone and muscle development. We do a lot of walking, trotting at various speeds. I also think it helps horse learn she does not NEED to always have her head in the lead, teaches rating keeping her head by your knee on a slack lead. You need to be able to rate her later on, not have her get agitated by someone passing or ahead of her in competition. She can waste a lot of energy fighting rider who wants her to stay back.
Breed prejudice here, but keep an eye on her QH side for hoof issues. Hoping she got good Welsh bone and proportionatly larger hooves to body size. The bigger, deeper hooves seem to help keep a horse sound longer, take the miles of use. Best of luck having years of fun with her!!