I recently read something by a local running coach that said "conventional wisdom is that 80% of your training should be easy-moderate, and 20% hard, and that's actually backwards if you want to get real results! You need to train HARD and FAST more often!"
You reply, "Sounds great, in ten years tell me how you're going with that."
There are a lot of hard diets and hard workouts. Quite often, they work! for as long as you can do them, a few months at most. Now, if you're working out for That One Day, like a competition or wedding or something, that's fine. But... there's a reason there are no Biggest Loser Specials. If you're working out for your health - well, you need something you can do for a lifetime.
Okay, this is my general advice to people. Individual needs vary, it depends on your capabilities (past training, injuries) and goals. So the "you" is the generic you.
The first thing to look at with endurance is a screen. If you have an ankle, knee, hip or lower back issue, see a physiotherapist or the like - don't start running. Find a BMI calculator online, if you're over BMI 35, see a doctor before doing anything, because you're carrying a lot of extra pounds - just think, if you got a healthy bodyweight but sedentary person and got them started running, would you advise them to carry an extra 50-100lbs with them while they did it? And if they did, what would you expect the result to be?
So if you're BMI 35+, see a doc. Likewise if you're BMI <18.5 - you may have low bone density or something, you don't want to be pounding the pavement. Long-term, nobody BMI >35 or <18.5 is healthy. Sorry.
If you're BMI 30-35, start with a 30-60' walk each day, don't worry about heart rate or whatever. Keep doing that and making the necessary dietary changes to get under BMI 30. "Yeah but it's all muscle." Probably not. But even if it were, that's still weight through your joints as you pound the pavement, so start with a walk.
If you're BMI 18.5-30, then you should still start with a 30-60' walk each day. Do that for about three months.
Then start running. But here you want to look at the
Maffetone number. This is an easier pace than you think. For most people it's going to be a slow shuffle barely faster than walking pace. That's fine! Over time you'll need to go faster to hit that number.
Many people have used this and similar methods to improve their cardiovascular fitness. But most people try to make up for a decade or more of being sedentary by going hard. If it doesn't injure you (and it does injure a lot of people) it works! But you can't keep it up.
Because also - I think it probably varies a LOT on the person, and of course, individual risk of injury. Two years ago I ran 3 half marathons and I trained HARD and made a lot of progress. (Not intentionally that hard, but it occurred to me AFTER the 3rd half marathon that I'd been running all my long runs at race pace, oops!!)
Last year though...every time I tried to ramp the intensity (speed or distance), I got these nagging injuries.
People are fairly evenly divided into meatheads and wusses. Meatheads go too hard and hurt themselves. Wusses go too easy (if they show up at all) and never achieve anything. A good programme, trainer or coach will help you steer the middle path. If you hear "go hard or go home" or the like, they're a meathead, avoid. If you hear "bosu ball" or "pilates" they're a wuss, avoid. Look carefully at the people they train. What have they achieved? How many injuries have they had to work through? How long have they worked with them? Be sceptical.