Is it possible to run and do an hypertrophy training at the same time and don't lose weight? I am currently training for 20km: 3 times a week, but since I started, I lost weight.
Absolutely. First, why WON'T it happen?
Aerobic activity following Strength training impedes the mTor pathway. What the heck does that mean? It means if you lift and then run, the hormones responsible for hypertrophy will be inhibited and prevent mass gain.
When strength training, rest periods greater than 3 minutes allow the ADP-CP system to refill which also can prevent hypertrophy (ok, that is oversimplified but the idea is long rest prevents hypertrophy).
In order to achieve hypertrophy while also running...
1) Run first and then lift
2) Lift more reps with less rest between sets. Limit rest between sets to less than 2 minutes and usually a lot less. Try working 3 by 10 with 45 - 60 seconds between sets.
And be sure to keep your calorie intake up but focus on nutrient dense food sources (skip the junk food).