Overtraining

Training for a marathon implies that we do a lot of long runs.
A long run is a greater burden on the body. We must be careful that we do not overtrain.

These are warning signs:
- unusually intense soreness
- slowed and poorer regeneration and elevated resting heart rate
- Legs feel heavy and sluggish
- persistent fatigue and performance decrease
- increased irritability, moodiness or depressive moods
- general motivation problems
- appetite changes (more or less)
- inability to sleep deeply

To prevent this there are 8 Ways to Extend Your Long Run | Runner's World.

If you want something you've never had, you're gonna have to do something you've never done! Right or not? The following studies suggests always to self-monitor in case of taking risks:

ncbi.nlm.nih.gov - Self-regulatory skills: are they helpful in the prevention of overuse injuries in talented tennis players? "Possibly, self-monitoring can help players to prevent themselves from time loss overuse injuries.".
ncbi.nlm.nih.gov - Is risk-taking in talented junior tennis players related to overuse injuries?: Taking risks creates the risk towards overuse injuries.

See also:
outsideonline.com - Don't worry about exercising too much: "Too little exercise is way riskier than too much".
outsideonline.com - There might finally be a cure for overtraining: Ketone supplements

outsideonline.com - acute-to-chronic workload ratio (ACWR)