Gained Muscle but didn't lose fat?

I had been running for several years prior to training for a marathon, so I guess I lost any weight I was going to lose. I need to keep eating while I am training to keep up my strength/energy and feed your muscles to be able to run. I don’t know what am I doing wring, the thing is I gained muscle but I didn’t lose fat.

I ask my friend who is training triathlon and he said that runners also gain weight because their blood vessels ticked and you are probably drinking lots of water to keep hydrated, and water weighs a bit too Another reason could be that you gained muscle mass and didn’t lost the fat is because the muscle is denser than fat, so a certain amount of muscle weighs more than then equal amount of fat, but if you replaced your muscle with fat, you would actually gain weight on the scale but be stronger and healthier.