I loved the AT through NH, although it is less about hiking and more about carefully putting one foot in front of the other to avoid tripping and falling on an exposed body part. Consequently, you average speed will plummet from what you could do on a smooth trail, regardless of elevation change. Hikers frequently average a full mile-an-hour slower on the NH AT versus the southern AT. Hard to believe, I know. I actually found that I was almost as fast going uphill over boulders than along Kinsman Ridge or down from the summit of Mt. Garfield.
I hiked from Glencliff to Pinkham Notch in September 2006 at the age of 49 and in excellent shape:
Day 1: Glencliff to Beaver Brook Shelter (7.9 miles; +3900' ascent/-1200' descent = 5100' total vertical (646'/mi))--Pretty easy hike but a good slog over Moosilauke. Took me about 5 hours with a need to wait for a T-storm to pass over the summit.
Day 2: Eliza Brook Shelter (9.1 miles; +2300/-3800=6100 (670'/mi))--The descent to Kinsman Notch was brutal, and I took it too quickly with little rest, over-stressing my quads early in the hike that led to future knee pain. The walk along Kinsman Ridge was surprisingly rough and slow. It took me about 8 hours, and I'm glad that I decided to stay at the shelter rather than push up South Kinsman.
Day 3: Liberty Springs Campsite (11.4 miles; +5350/-3300=8650 (759'/mi))--Brutal climb up South Kinsman, even though I was rested. Long, hot climb up the river of boulders to Liberty Springs.
Day 4: Galehead Hut (10.4 miles; +3250/-3250=6500 (625'/mi))--Very cold morning, but a beautiful day with little breeze for the walk along Franconia Ridge. I think I spent almost 2 hours atop Lafayette taking in the view and eating lunch.
Day 5: Crawford Notch (14.7 miles; +2000/-4500=6500 (442'/mi))--This was probably our easiest day, with smooth trail after Zealand Hut
Day 6: Zero Day waiting out the weather
Day 7: Lakes of the Clouds Hut (11.4 miles; +4600/-900=5500 (482'/mi))--A long but beautiful climb up Windsor Cliffs, even though it was pretty foggy for me. We were in the clouds all the way to LOTC Hut.
Day 8: Half a mile trail-south of the Auto Road (11.4 miles; +3000/-5400=8400 (737'/mi))--An absolutely gorgeous day for summiting Mt. Washington, with an undercast and absolutely no wind. We lollygagged across the Presidential Range, just enjoying ourselves and wishing we could keep hiking.
Day 9: Pinkham Notch (2.4 miles; +200/-800=1000 (417'/mi))
In September 2012 I completed NH on my way north to Katahdin as a slightly less fit 54-year old with balky knees:
Day 1: Pinkham Notch to Carter Notch Hut (5.9 miles; +2700/-1400=4100 (695'/mi))
Day 2: US-2/White Mountain Hostel (15.2 miles; +3600/-5900=9500 (625'/mi))
Day 3: Gentian Pond Shelter (11.8 miles; +3600/-2300=5900 (500'/mi))
Day 4: Full Goose Shelter (9.6 miles; +3400/-2600=6000 (625'mi))
You can find my trail journal at http://www.trailjournals.com/kerosene, with the start of the 2010 hike at http://www.trailjournals.com/entry.cfm?id=262021