What my plan doesn't take into account

My plan leaves a little leeway for setbacks, but it's awfully early to have had one! Today was supposed to be: run 3:15, walk 1, for 32 minutes. It was also about 18 degrees outside. After six straight days of some kind of workout or other (though some just swim or yoga) my legs felt like lead from the start. I hadn't had a puff of Advair in 2 days (it's lost in my mess somewhere) so I started gasping for air. I made it through the first couple of intervals, and after that it was more like run 2, walk 1:3o, run one, walk two... I kept telling myself that some workout is better than no workout, but I remember this from last year - my body just wouldn't follow the plan. I know rest is good - I'm all for rest! But I also wonder: is it a good idea to stick to my schedule on the theory that I'll toughen up and get used to more frequent workouts, or go easier, take more days off, with the idea that I run better when I feel fresher? Tomorrow I COULD go for a morning workout class at my gym, but the Friday morning teacher is TOUGH - it will whip my ass. I'm going to spend most of the rest of the day walking (I'm playing hooky in NYC for a day - yay!) so I'll be active. I'm tempted to skip the class, do a little stretching and ab work at home before I leave, and go back to Pilates/elliptical Saturday, long run Sunday.