I really like it. (Although it took me a while to catch on that Akari was the girl, hehe.) You portrayed the narrator's longing and eventual release very well. The letters which consisted of only one sentence worked particularly well in reflecting the bottled-up emotions and restraint. And I liked the sketches of setting; they provide enough poetic detail without turning a letter into an essay. The one thing that I personally didn't like was the lack of originality in places. For example, the sakura and the scene where Akari is married to some other man are... overused, in my opinion. Maybe I've watched too much anime. Although you did spice up the sakura using the 'five centimeters a second' thing.
I enjoyed that. Oh yeah, and I like how Tohno doesn't use his name at the end. It's like he's decided that it doesn't matter if Akari never remembers his name again. Good job.