走ったほうがいい　you have better run. What you have two choices- have better run or not and choose one, Japanese use this phrase. 走るべき　You have to run. It’s used when you would like to make someone run. We don’t use his phrase normally when you talk to your friends. 走ればいい　That would be good if you run. 走れば　is conditional form of 走ります. When you want to tell the condition, we use it. Eg) Your friend is running late to meeting and he is upset. He asked you what he should do now. You have many choices to make it to meeting - taking a taxi, calling his boss, running to his office etc. in this situation, you could tell him 走ればいいよ(It means you can make it to meeting if you run.) I hope my answer will work your study!