Low drop run goes above 11km

Previously I've worn the low-drop Brooks shoes up to 8km and today I wore them for an 11.4km road trip trough Dore village. The shoes do feel 'fast' and indeed I recorded some of my best times in months but during the last few km of descent I could feel tension in the calf muscles on both sides. That's an indicator that I must be careful about lengthening runs in these shoes. Although they are a long way from 'barefoot' the heel drop is a few mm less than my usual road shoes.