For me low carb and ketogenic are basically the same thing on a sliding scale - if I'm eating few enough carbs, then I produce ketones, and am burning both carbs and fats as fuel. Which means I can keep my glucose levels lower and stable, and if required maintain or lose weight by adjusting the amount of fatty food I eat. And I certainly
don't eat excessive amounts of fat - I've always eaten normal full fat food, and have just upped consumption somewhat to compensate for my reduction in carbs. As
@lessci has said , it's essentially a matter of eating adequate fat to satisfy any hunger (and I rarely feel hungry at all) , so it doesn't necessarily
have to be a massive amount.
20-25g carbs is considered the guaranteed level to achieve ketosis, but once you've been at this level for long enough, your body will have (re-) adapted to using both carbs and fats for fuel, and it's then often possible to be a little more lenient with carbs. I generally aim to keep under 50g carbs a day - consumption is currently based on experience - I don't count or eat the same amount all the time, but will use my meter to keep an eye on glucose levels, as it's not only diet that can affect them.
When I was diagnosed I simply cut right back down (again) on carbs, and saw an almost immediate reduction to pre diabetic levels, and then at GP's suggestion for weight loss, based on Atkins induction phase (very low carb/keto) for a few weeks, lost around 20 kilos, and as a bonus reduced glucose levels further to just on normal, where with a few non food related glitches they've remained....
So, as to sustainability - I've kept my glucose levels down to high end of normal-low pre-diabetic for seven years, kept my weight down & stable, and for the last three years haven't required any diabetic meds - all without any severe restrictions on my diet.