SG - shows how different we all are for preparing for a race. I wouldnt go out for any run the day before a race let alone a double (even though they are minor distances) yet i know others that its essential for them. My coach for example does the same as you too.
This Nike store ad is getting on my nerves now!
I'll always do a short, easy run the day before a race, as otherwise the legs seem to stiffen up. usually rest on the Friday. Not sure I'd do a double though
Weather forecast is looking very dodgy - very wet, but biggest risk is getting stuck on the way back in snow, with some forecasts predicting 4" of snow in the Chilterns later in the day! Best make sure to take some warm clothes and food!
With HR Philip, isn't one of the reasons to use them to know when you have an elevated heart rate through being ill and adjust your effort accordingly? The coffee point is interesting - I always have a strong coffee before a morning run! the biggest issue for me with a HRM is the thought of wearing the chest strap....