I worked for a few years as well, ending five years ago. It's amazing how hard it is to translate the experience of that job. It's a bizarre combination of physical labor, customer service, and office work, any one part of which is sort of boring after a while.
I still spend a lot of time cycling, and am continually amazed by how
a. long bike parts last when you aren't riding 40 hour weeks (MKS steel toe clips don't usually wear through in six months, for instance). I have to remember to check things periodically to see if they're wearing out, rather than automatically forking part of each paycheck over to a bike shop.
b. how completely "use it or lose it" messengering-specific cycling skills are. I commute by bike now, and log about ten or so hours on my road bike each week, but I've completely lost the ability to casually zip into and through traffic. My basic handling skills are good, and I haven't lost street sense, but the higher-order things I used to be able to do? Gone.
I definitely miss the social aspects of the job, and of not taking work home with me, but, as people have said, it's a good thing to do for a while and then head off to new things.