| What happens when your run and take the lantus? Do you suffer from hypos? Is there a difference on rest days, do your levels rise without the lantus? It maybe that you need very little extra insulin because of the exercise but that you need a bit more on less active days. Exercising muscles utilise glucose without insulin and this effect continues after you stop thus greatly reducing your insulin needs overall.
My weekly mileage is less than half yours but running (or plodding in my case!) has caused a huge reduction in needs. Before runs I've been advised to reduce (drop for longer runs) my insulin at the meal before, to reduce it at the meal after and reduce my basal (lantus) on days in which I run more than 10 miles. BUT if I don't run for several days my insulin requirements quickly climb.
Personally I think a low carb diet is a bad idea, most sports nutritionists (as do most mainstream diabetes specailists) advocate a high carb percentage diet,( another piece of advice I was given was to eat slow acting carbs before a run) you do need to fuel those long runs and to restock glycogen stores after running.  | 
