Get tired too fast

I'm 16 years old, 6 foot/183cm tall and I'm 150 pounds/68 kg. I'd also like to state I'm very unfit.

I've been going to the gym since the start of the year, 3 times a week (Monday, Wednesday, Friday) and stay there for an hour. My main goal is to achieve greater stamina, as I currently become fatigued after simply walking for a few minutes at a normal pace.

A few weeks ago, I attempted to do 30 minutes of exercise on a stationary bicycle daily for 2 weeks.

I have not yet noticed any difference. I test my stamina by the amount of time I can jog at 10.5km/h on a treadmill, which has been at almost exactly 2 minutes all year round.

Am I just not training hard enough? I'm in the 140-180 heart rate zone.
Am I not doing enough? Unfortunately the gym is too far away to go daily, but I do exercise at home for 30 minutes on a stationary bike.

I've had a blood test with normal results and gone for a medical check-up which returned normal.

Ofcourse I understand my diet would be important, but I belief it's fine as well. It basically consists of a normal breakfast, a sandwich and an apple for lunch and meat/fish with salad in the evening. I don't consume fast food.

I'm getting very frustrated as it's been well over 30 weeks total and am becoming uncertain whether 30/60 minutes a day is enough. I can't seem to get fitter or loose any extra fat.