Marshal - I don't have any ultra-distance experience but I injured my Achilles thirty-odd years ago and it flares up from time to time. The thing that helps me most is getting my foot further forward on the pedal than many cleated shoes allow, which reduces the strain on the Achilles. I don't recall your setup (single-sided SPDs maybe?) but if you're using cleated shoes you could try changing to flat pedals (and suitable shoes if necessary) so you can get that pedal back towards your heel. All the best - you're an inspiration to those of us in the over-50 ranks.