If i'm not over weight and i walk for exercise is it bad to eat a lot ?

Depends: It depends on what you mean by a lot. Some people have a very active metabolism and never seem to put on weight. You are young and have an active metabolism. You can probably get away with a lot now but habits you develop now carry on and as you get older it is very hard to break those habits. Your metabolism will slow down later and you may find unwanted weight gains.

Balance: The major issue is related to how calories ingested related to calories burned. If you take in more calories than you burn - you will gain weight. If you burn more calories than you eat - you are likely to lose weight. The exception is that muscle weighs more than fat - thus if your walking leads to ^'ed muscle - you may gain some weight even if you become more slender. Take care.