Beginner to a great runner.

Hello! Recently I’ve been going to the gym consistently and doing full body for 3 times a week however my work has recently announced that later this year there will be a 26 mile walk that we’re doing on mountain/hill terrain. I’ve put my name down for it and it’s made me change my mind a little bit about my training. I think I’d prefer to be lean and have good endurance, stamina and speed than just build muscle and look like a body builder. Can someone please recommend an exercise for 3 days with a 4th optional day. I’d like get really great at running, and potentially even be good on the track too. I wouldn’t mind growing a bit of muscle all around too but not too much.