@hippy - You'll be racing and leaving your legs on the cold ground?
I'd keep the full length to start and experiment with moving your lower half down until you reach your comfort limit, especially given your experience.
That was my plan. Well, depending on when the mat arrives - if it doesn't in time then I'll try my Thermarest first. I have a cosy balcony sleep over planned soon. Might have to give the spiders notice before I move into their home.
@hippy - You'll be racing and leaving your legs on the cold ground?
I'd keep the full length to start and experiment with moving your lower half down until you reach your comfort limit, especially given your experience.