I've never done a 25k-minimum bang (the ones I've written in have had minimums in the 10k-20k range), but my best advice is to get started early. Procrastinating long fics is the worst thing you can do to yourself. I generally tried to write about 1k a day, more if I was in a good writing groove. That usually meant I'd finish ahead of schedule, which gave me and my beta plenty of time to work on tightening the fic up, doing any needed re-writes, ect. After all, they're a big project for the beta, too.

I am the wrong person to ask about outlining and planning, though, since I do neither. I'd probably be able to beta for you if you like, but could I maybe get a little more by way of specifics about what you're planning before making that call? There are some characters I'm useless with.