There is something about the gym that strikes people one way or another. Some like how it motivates them and gets them out of the house. Others can not bear the idea of working out in front of others and view it as a hassle to have to leave the house to get a workout in. If you are in the second camp then refer to the Beginner and Intermediate strength training for runners plan that can be done at home.
Something to remember at the gym: you are a long distance runner and/or marathon runner. That means that you want to train in a way that is going to enhance your performance and not detract from it. That being said, your focus should be on muscular endurance work.
A lot of gym-goers get wrapped up in the Olympic lifts which can be extremely beneficial to specific athletes. However Olympic lifts (like the jerk, clean and snatch) are more power moves. These would work great for you if you were a sprinter and needed that power. Since you are an endurance runner you want to focus on what will increase your endurance and fatigue resistance and this is done through muscular endurance work.
So in order to do muscular endurance work you should follow these guidelines:
Is this really important? Actually, yes, as physiologically, different adaptations are made depending on sets, reps and rest and what physiological adaptions do we want to see as a long distance runner? Yes, muscular endurance adaptations!
On a side note: Something that makes keeping track of all your repetitions and sets and where you are at in your weight lifting workout is the nifty Gymboss which can also be used as an interval timer!
It is great for beginners since machines don't allow you to do the wrong movements. So the risk of injury is lower.
However it is not as functional for muscles since most machines don't really target sport specific moves. Really you have to find what works for you and what motivates you in your strength training for runners plan!
Along with these machines do the exercise in the strength training for runners beginner and intermediate strength training for runners plans which can be done at home or the gym. These are mainly done with dumbells and resistance bands. Some can be done with barbells as well and are completely doable at the gym!