"Andrew, I am a distnace runner and have a long and lean frame. I want to build muscle or at least have more muscle definition but don't want to cut down my milage. Do you have any suggestions for building muscle for a distance runner?"
Here is a short, sample beginner training plan that could be used by a runner who wants to start doing some resistance training to build lean muscle:
Split Squat (progress to Bulgarian Split Squat)
1-Arm DB Press
Neutral Grip Pull-Up (machine assist or band asssist if needed to start). You can also use a doorway chin-up bar and one leg on a chair for assistance.
Weighted Hip Hinge (progress to dumbbell or barbell Romanian Deadlift)
Push-Up (start from the bar if needed)
Inverted Row, Prone Dumbbell Row or 1-Arm Dumbbell Row
DB Toe Raises
Side Bridge alternated at each session with Front Bridge (20-90sec hold)
This can be done 2-3x per week. Do 2-3 sets for each exercise with about 8-12 reps. Start light and work on technique. Be sure to get some coaching as needed to learn the movements. Once you have the technique down, work on gradually, but consistently adding weight. You can also add a few accessory exercises (e.g. something for abs).
A question I would have for you is why do you not want to cut down on your mileage? The reason I ask is that you kind of have 2 conflicting goals. Distance running tells your body to lose muscle - this is especially true for those with a long, lean frame. You can try and it may work for you if you are a beginner, but if things are not working, then you have to re-examine your goals and training priorities. For most people who have what seems like conflicting goals, I recommend emphasizing progress in one goal while maintaining the other one. For example, if building some lean muscle is your goal, I would back down a bit on the frequency and mileage you run so you can devote more time and energy (remember, you only have so much recovery reserves) to that goal. On the other hand, if you are training for an upcoming race, then prioritize the running and back off the resistance training to more of a maintenance level.
All the best!