In August my friends and I hiked the JMT, doing 220 miles over 16 days...one of those days being a full rest day.
Things that I took from it was that elevation gain and loss, as well as altitude were more important factors than overall mileage.
For example, in 2009 we did a training trip for the JMT on the Tahoe Rim Trail, which was a relatively easy trail with not a lot of elevation gain or loss. This allowed us to kick out 15 to 20 miles easily.
On the JMT, on the 2nd half of the trip (Yosemite to Whitney, Southbound), facing a pass everyday at altitude was a butt kicker where we were averaging about 15 miles per day...much more difficult than the TRT.
Anyway, things to consider for a longer trip: Foot Care. It is all about your feet. Extra socks, taping feet, hot spots, etc. Same thing for chaffing. I found that my inner thigh was completely sore to the touch from chaffing and I needed to tape my inner thigh. Very uncomfortable.
I also agree with the prior posting....you will want extra food on a 2nd week.
I would also add that you might want to make a point of washing your clothing on the trail. I can not tell you how much I started to stink and going to sleep with the overwhelming stench of myself was getting old.
Monkey Butt...avoid it at all cost for keeping your sanity. I took to using Mike Clelland's tip on washing my backside with soap and water after depositing into a cathole.
At higher elevation, I found that my fingers were cracking and splitting open, sometimes bleeding due to the dry air.
Mentally, I found that having some nice treats or snacked in my resupply kits that we mailed out to ourselves was a nice pick me up. Be it a favorite candy bar, dried fruit, etc. In our case, having a zero day of rest at a resort, Vermillion Valley, allowed us to drink beer, eat hot food, and to shower.
Bottom line: On the longer trips, it is about taking steps to slow the eventual break down of your body and to keep your spirits up.
Hope this helps.