I think it's a brilliant idea. I find it much easier to motivate myself to run (or bike, or ski), if I have to get wherever I'm going. If I start at home and initially am going in the "wrong" direction, it's so much harder. Nothing like a boss at the other end to stop me from thinking "hm, do I feel like doing this today?"

I half-ran my commute quite a few times last winter. I took the metro half the way, and ran the rest of the way, 30-40 minutes run. The biggest hurdle was accurately guessing how little extra clothing I could get away with to stay warm before my run, as I had to pack it in a hipbelt and run with it afterwards. I didn't need to carry water, though. Funnily enough, the most annoying thing was running with small noisy things like loose change, my cellphone etc, not the extra windjacket that packed down nicely.

I think you have a great plan, especially the part about having a pannier all packed and ready to grab. I'd probably be messing around trying to re-use stuff from the day before. Let us know how it works out.