I'm still enjoying it, and my knees have even started hurting less. This doesn't wear on the body like lifting weights does.
I am starting a class tomorrow that will meet every Tuesday and Thursday for quite a while, so I'm either going to walk in the mornings or at lunch. I am going to set my alarm to get up tomorrow and walk super early, but if I can't manage it I'll walk at lunch. That'll probably be the sum of it.
As of right now, I still plan to double my walking to 3 miles a day after I hit 50 days. While the walking I'm doing right now does make me tired by the end of it, I find myself wanting more of a challenge. We'll see, though.