Dag; i assume you are talking about cycling, not running.
the problem i find with riding, is that there is a need for 'fuel' as you 'up the ante' in order to not 'drop off' so to speak.
If you are new to two wheels (at least the serious version) two important things: cadence, and water.
Stay away from the 'sport drinks' as they bugger up your blood sugar, stick to water, and a suitable source of converted sugar, most normal foods, in minimal quantities.
The cadence thing is to keep up a suitable rotation speed (probably slightly faster than feels normal) to maintain your heart rate, vs power output without over taxing your knees' ability to apply torque.
Longevity is the aim; speed will come.
Caution it becomes a way of life, and fits into the obsessive compulsive mindset most seamlessly!