I've been riding for a while and put in about 60 to 80 miles per week. I have lost weight and feel better but my stamina isn't where it should be. I hydrate well so that is not a problem. I've never completely bonked but it seems that my legs give out much too soon. Spinning is more tiring than mashing and while hills are getting easier, I lose power before the crest. I can't do a hill without breathing heavily but I can still hold a conversation.
You don't have to worry about bonking until you're doing 60-80 miles in a single ride. Even so, make sure your glucose-level doesn't drop too much during a ride, eat steadily to keep it up.
As for stamina, it'll come with more training; you might be expecting too many results too soon. You'll want to put in about 2-months of what's known as "base mileage", which is doing LONG rides at a steady-state speed, say... 16-18mph. Increase your weekly total about +10-15% a week until you're doing about 200-300 miles/week with 50-60 miles on a single ride. This endurance is the 1st step towards building stamina.
The 2nd part comes with doing intervals. These are short 1-5 minute efforts above LT where your heartrate steadily increases to max. It's the last half of these intervals where you feel like you're gonna gag up your lungs and your legs are gonna fall off from the searing pain that really builds up the stamina.
So the short answer is... more training. Have fun!
