Historically, true light infantry is not just a unit you drop into and settle with after basic training. They had their own selection process that attempted to weed out those who did not possess the right endurance needed for the missions. The biggest requirement that stuck out to me was conducting 5 mile runs with a full combat load and weapon. The runs started at a 10min/mile rate and the distances and the pace would increase daily, giving little time for recovery and adaptation. The point was finding capable people NOW, not forming people into what they wanted.
If you are wanting a current equivalent of these types of requirements, I know the British Royal Marines do this type of training, and even the regular infantry have a 2 mile run in full fighting order (combat load and weapon). The American military would not dare do something like that. Personally, I think it is out of weakness. The only time you see weighted running is the ruck trots people do in special forces selection. Granted they are carrying a lot of weight, but rucking 60 pounds for 12 miles at a trot once in a while and running in combat gear daily with an increasing pace are different.
Light infantry training included conducting training for days on end with limited water (one canteen per day in the jungle) and little or no food for a certain number of days (no one was told). This was to acclimate the body to the realities of combat where water and chow are secondary to the mission. Light infantry could be in the field for weeks at a time and need to evade the enemy nonstop. Part of that means leaving no spoor/tracks from sitting down to satisfy a grumbly belly.
Today you hear people talk all the time about sipping and guzzling water. In fact, science has shown this to be a bad idea when not balanced out. You can go for an amazing amount of time with little water, as long as you loaded up prior to stepping off. When I was in the Marines, I was taught that canteens are not sippy cups. They are either empty or topped off. This means you only drank water when the entire quart could be taken. On top of that, you better back that up with some food or electrolyte pills because water, water, water without nutrients will cause dehydration symptoms even in low intensity activity. I’ve seen that plenty.
Light infantry units were often well known for their fieldcraft expertise. They had to be exceptional at things like advanced camouflage techniques, tracking/anti-tracking, caching, infiltration, land navigation, night navigation/patrolling, ambushing, raids, etc. Lots of the light infantry training in fieldcraft mirrored what SF was learning because it often operated behind enemy lines on its own for days or weeks alone. Light infantry was trained more like a special infantry. Their strength was their speed and fieldcraft. Conventional tactics were not really on the menu for light infantry elements, which gave them such a hard punch against the enemy. They would sneak in, sucker punch some rear echelon units, destroy supplies, and leave without being seen. Then they would do it again a few miles away in a day or two, often with little or no support.
Based on the historical performance of light infantry, I don’t know of too many light infantry units in the western world anymore. You see people playing light infantry at Ranger school, but then they armor up and return to being heavy line infantry after school is out. Even the Europeans are hypocritical in their training because they run without armor in training as light infantry and then armor up and ditch their training on deployment. But then again, light infantry is a thing you see in militaries that use maneuver warfare. Those that practice attrition warfare have little need for maneuverability. A sad truth only true students of history will understand. If your ego/identity is wrapped in military service, you will be feeling offended right about now.
It is sad to see, but attrition warfare does own the hearts of most western Generals while our enemies in Vietnam beat our ass using maneuver warfare. Instead, the Western military, with their endless pit of taxpayer dollars continues wrapping our forces in 30 pounds of ballistic bubble wrap instead of investing in updating our WW1/WW2-era fighting techniques. I guess it is supposed to make moms feel a little better about their sons dying. At least they did all they could to protect precious Johnny, except bring back light infantry so we can win against our next enemies. Other than that, I have no opinion.