Runners like to run.
However, from my experience, sometimes it is good to introduce other types of training into our schedules. If we always do the same thing we will usually get a similair result.
Strength training is important as :-
1) it allows us to increase the amount of running we can do
2) a strong muscle is less likely to breakdown and get injured
3) a strong muscle can do a simple and repetitve activity for longer i.e. long distance running
If you have access to a gym, use it, and work on the upper body as well(many long distance runners complain of aching arms and shoulders). Use heavy weights with no more than 10 reps per set and 3 sets. Use free weights where possible as this will bring into play more muscle groups(stability) and encourage different muscle groups to work together, as muscle groups do when running. It is important to use free weights where possible and especially for exercises such as squats, lunges and walking lunges.
Examples of set/rep mixes will include 3x10reps, 3x8reps, 3x6reps, 1x10+1x8+1x6 or 4x4-6reps.
Strength in your legs will make every step feel easier.
See this video for a routine.