Dan has great advice: start small and work into it... and just get out there and see how you do!
My additions: I find that Day 3 or 4 is always a killer for me -- so I try to plan accordingly now. Once past that I get a good second wind. I've also learned to try and make Day 1 short -- not because of fatigue so much as I tend to always be late w/ last-minute organization, permits, etc. I like to start a trip in the early afternoon if possible.
Also... Take care of your feet! I really thought I did on my last hike, but alas I didn't take enough care for a blister. Minimize walking with wet socks (I love neoprene while in water) and address any hot spots early. I tend to always get a blister (or three), but with proper treatment they will usually just callous over in a few days.
And... know your pace across different types of terrain. I'm guessing you already know this, but 15 miles through the MAZE is so much harder than 15 miles along a trail in the mountains. Water availability will also greatly impact one's speed.
Lastly... know what you like and don't feel the need to race for mileage or bag peaks or other "trophies" (unless you decide that IS what you like!) I find that I'm different than many other hikers in that I hate hiking in the early morning, but instead will hike until sundown almost every day. Also, I don't stop for a big lunch, but instead take several snack breaks throughout the day. And lastly, I try to never be in too much of a hurry to do side-hikes or go out of the way a bit to get a better view and take the time to take pics/video. Same goes for gear. I try to go as light as possible, but there are some items that I just refuse to give up because I'm happier with them.
One more thing... learn how to get and be ok with taking time "off". When I started planning my first 3-week outing, it was a tough sale for the boss as well my girlfriend. BUT, I've learned to make the most out the time before/after my trips to make it ok on both fronts.
Good luck!
- Jamal