If he isn't already eating a VLCHF diet, now's the time to start; as a T1 diabetic, his chances of doing a carb-fuelled ultramarathon are precisely zero. I assume your own diet is heavily fat-based anyway? His main problem, I reckon, will be joint issues. I run a 5k two or three times a week and I find that, although it's not especially tiring (27-minute pace), my hips and knees get a bit creaky.
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