Breathing:

Regular natural breathing in rhythm with the running movement improves cardiovascular activity (endurance). Please note:

The body regulates its oxygen requirements automatically. Breathing adapts automatically to the walking rhythm. During exertion, the body gets the air it needs by breathing faster. A "forced" breathing rhythm should therefore be avoided.
Breathe in through the nose and out through the mouth.
Pay particular attention to deep, complete exhalation. This ensures improved exhalation of carbon dioxide (abdominal breathing).

Overexertion:

Overexertion should be avoided at all costs. It is important to interpret the body's signals correctly and deal with them. Never run faster than an increase in performance of at least 10% is possible at any time. A good training effect is achieved if you train regularly (e.g. 3-4 x 30 minutes per week). Only one, longer training session per week is less recommended, as it could lead to overexertion and have a counterproductive effect.

Pulse control:

The following rule of thumb applies to the normal amateur athlete:

Training pulse = 220 minus age, 70-85% of which (= pulse target zone)

So, for example, a runner with a pulse of 20 years must train with a pulse of at least 220.For example, a runner aged 20 who wants to improve their fitness level must train between 140 and 170 heart rate beats.

To check endurance capacity, the difference between the training heart rate and the recovery heart rate (immediately two minutes after the end of training) should be measured. Differences of 60 or more beats indicate a very good level of endurance. Note: "The higher the performance heart rate, the greater the difference in heart rate should be".

Side stitch:

Side stitch is not a disease. Occasionally, you feel a stitch-like discomfort on the left or right side of the abdominal area just a few minutes after starting to run. The actual causes of this common phenomenon are still not exactly known. Some studies suggest that this may be a purely mechanical cause. The connective tissue that supports the abdominal organs is under much greater strain after a meal. Physical activity at this time can cause small tissue tears and bleeding. Other explanations include insufficient oxygen supply to the diaphragm or pain caused by blood redistribution in the spleen or liver.

Possible measures to prevent side stitches may include:

Training the abdominal muscles; weak abdominal muscles increase the risk of side stitches.
No solid food intake (1-2 hours) before training or competition. Liquid food (non-carbonated) or very easily digestible food is recommended.

Possible measures for side stitches:

Interrupt training and press inhaled air into the lungs for 5-10 seconds in a crouching position (repeat the process several times).
Continue running at a reduced pace and change your breathing rhythm until the pain has disappeared.