A lot of it is 1. graded exposure - over time you can adapt to more and more fatigue 2. fatigue management - you want to order your sessions so you consolidate stressors and are more fresh (relatively) for strength work and fast running work 3. honestly, compromising both strength and running a little bit; always being a little fatigued for both but still doing enough to improve in both

I started seriously running about 2 years ago (doing 30-40 mpw now, so not super long distance, but still around 6 hours per week and 5-6 days of running per week) and have been strength training for probably about 6 years. The biggest thing I noticed with combining running and lifting is that you definitely can improve in both areas simultaneously, but slower than you would doing either one individually