In answer this:

So, assuming a person of say, 35, with an average weight and in reasonably good health is running a moderate pace on a flat surface, where, on average, is that point at which s/he needs to increase intake?

If you are in good health and running less than 15km in a single session I wouldn't bother with a special diet. Humans literally evolved to travel long distances on little food. If it's less than 25km I wouldn't even bother with special mid-race meals or gels. A weekend warrior who isn't aiming to place should be able to do a half-marathon (21km) on a moderate bowl of pasta the night before, a small bowl of white rice about 1-2hrs pre-race and whatever sugary electrolyte drinks are around the track.

If you are the kind of person that considers a light run to be an excuse to "refeed" it's unlikely that you actually need any special pre-, mid- or post-training nutrition asides from your regular meals.

In answer to this:

So, assuming a person of say, 35, with an average weight and in reasonably good health is running a moderate pace on a flat surface, where, on average, is that point at which s/he needs to increase intake?

If you are in good health and running less than 15km in a single session I wouldn't bother with a special diet. Humans literally evolved to travel long distances on little food. If it's less than 25km I wouldn't even bother with special mid-race meals or gels. A weekend warrior who isn't aiming to place should be able to do a half-marathon (21km) on a moderate bowl of pasta the night before, a small bowl of white rice about 1-2hrs pre-race and whatever sugary electrolyte drinks are around the track.

If you are the kind of person that considers a light run to be an excuse to "refeed" it's unlikely that you actually need any special pre-, mid- or post-training nutrition asides from your regular meals.

In answer this this:

So, assuming a person of say, 35, with an average weight and in reasonably good health is running a moderate pace on a flat surface, where, on average, is that point at which s/he needs to increase intake?

If you are in good health and running less than 15km in a single session I wouldn't bother with a special diet. Humans literally evolved to travel long distances on little food. If its less than 25km I wouldn't even bother with special mid-race meals or gels. A weekend warrior who isn't aiming to place should be able to do a half-marathon (21km) on a moderate bowl of pasta the night before, a small bowl of white rice about 1-2hrs pre-race and whatever sugary electrolyte drinks are around the track.

If you are the kind of person that considers a light run to be an excuse to "refeed" its unlikely that you actually need any special pre-, mid- or post-training nutrition asides from your regular meals.

In answer this:

So, assuming a person of say, 35, with an average weight and in reasonably good health is running a moderate pace on a flat surface, where, on average, is that point at which s/he needs to increase intake?

If you are in good health and running less than 15km in a single session I wouldn't bother with a special diet. Humans literally evolved to travel long distances on little food. If it's less than 25km I wouldn't even bother with special mid-race meals or gels. A weekend warrior who isn't aiming to place should be able to do a half-marathon (21km) on a moderate bowl of pasta the night before, a small bowl of white rice about 1-2hrs pre-race and whatever sugary electrolyte drinks are around the track.

If you are the kind of person that considers a light run to be an excuse to "refeed" it's unlikely that you actually need any special pre-, mid- or post-training nutrition asides from your regular meals.