wdypdx22's "HITT" or "interval training" might be what a nutritionist who I talked to called Fartlek training, in case you want to Google that term too.
afterwards if I should cycle further I'm exhausted
I don't know that it's good, nor even possible, to 'train' when you're 'exhausted'? So, why are you exhausted, and how to avoid it: perhaps you need to rehydrate or to refuel, perhaps on the fly; or perhaps your 'normal speed' for the first hour needs to be slower/easier (even for the fittest, the 'normal speed' for a sprint isn't the same as for a marathon).
Apparently a 180 lb cyclist at 12 mph (on the flat, presumably) burns about 500 calories/hour.
FWIW I've been cycling for a bit over an hour twice a day, 3-5 days/week for the last month (after eating, so I don't run out of fuel on the way). And I haven't been pushing myself too hard (e.g. I'm using low gears when I get to a hill) because I don't want to strain my knees: and I'm sure I'm getting stronger/more enduring.
If strength is about increasing muscle, perhaps endurance is about increasing vascularisation. Apparently when you exercise your "VO2max" i.e. the rate at which you can burn/use/take in oxygen increases, and that's partly because you get more capillaries (I imagine, I don't know, that it's blood vessels that were previously too small to be effective that increase to meet the demand ... perhaps it's new vessels too); maybe that (growth and change in tissues) can't happen overnight.
So whats the best way to improve my stamina? Should I practice every day a little bit? Or is it better to sometimes go for long tours?
Apparently someone once asked a champion whether it's better, in a race, to push hard and slowly in a high gear, or to spin more easily and quickly in a lower gear: and he thought about that and said, "It's better to push hard and quickly in a high gear."
Similarly (and here again, I'm guessing) improving stamina might not be to do with practicing either often or for a longer time, but both.