I just finished my fall training segment where I focused on a marathon in October and then 15k and "shorter" stuff after that. I just took my time off and I am interested in putting in a base building block where I'll run a couple consecutive months to just build endurance. No crazy workouts. I want to hit 75-80 miles a week. Should I focus on 6&6 doubles and 8&4 doubles or should I focus on longer 60+ min. runs but less mileage (65-75) to build endurance during base-building . I plan on doing easy days at 7:05 min pace with LR of 15 to 18.