How much it sucks when I let it go.... its a journey, not a destination - therefore there is no stopping .. just various potty breaks and time to stretch the legs or get a nap. Half the fun is just seeing where the road leads and pulling over to the side of the road to get some shots of the scenery!
In terms of "off" days - that depends on if its "off" because my body is telling me it needs a break. But usually I know I'll feel that much better when its over if I get into the gym when I'm just having a shitty day. Sometimes just being able to plug into 90 min of my favorite heavy metal is enough to make me feel better. If I can throw around heavy stuff too, even better!
Generally what got me in the gym & keeps me there is the fact that I can go in and always challenge myself and accomplish something every day - if its more reps, higher weight, different exercise I've wanted to try, or just the plain old physical exertion - esp given that I drive a desk for a living.