I just started easing myself back into the habit at the end of last month with some shorter walks. This week, though, I've been walking to and from work (5 miles round-trip) every other day. On the off days, I do my exercise routine in the morning (I'd walk in afterward, but I never get up early enough), and walk home about half the time (usually depending on whether I remembered to bring my sneakers to work).
I feel so much better. I'm more awake and alert when I get to work, and usually in a better mood, since I had a nice walk with my audiobook instead of fighting for a seat on the T. And by the time I get home, I'm over anything that was bothering me at work. Plus, I can get up half an hour later on walking days than exercising days, so it feels like I'm being lazy.

I finally feel like myself again!
The trick now is not to overdo it.