I'm too am looking at a JMT hike for this summer with my GF. It'll be a late season hike for us though, starting a few days after Labor Day. We'll have about 10 or 11 days, so we'll need to decide if we want to push it and do the whole thing or break it up into two section hikes (~half this year, ~half the next).
We'll do a few shorter trips earlier in the summer throughout the Sierra to see how we're feeling and decide what sort of mileage we feel like keeping up. 20-25 mile days aren't really an issue for us, it's more about how much leisure time for fishing, photography, etc. we want to take advantage of as compared to time on the trail.
Either way, it'll be fun and I'm looking forward to an extended trip... all of these 1-3 night trips the last few years have just been a tease.
If any of you are planning a late summer trip, maybe we'll see you out there.