Welcome to our forum. Speyside is a good choice for a first long-distance walk, but camping definitely makes it much harder unless you use a baggage transfer service. However fit you are, carrying heavy loads (tent, sleeping gear, cooking stuff presumably heavy) puts a lot of extra strain on your feet and if it rains (always likely in Scotland) you'll have nowhere to dry things out if wild camping.
By September, you'll have only about 12-13 hours of daylight. Anyway, most of our walkers look on anything over 17 miles as a long day. So 25 miles sounds over-ambitious to me - unless you plan to jog.

If you haven't followed a long trail before, let alone practised carrying heavy loads, I'd suggest you consider 12-15 miles long enough for a day's walk.
But then I'm a softie, and I'd go for a nice warm dry B&B if I possibly could! Can any campers advise further?