This is the classic trade off of wanting to have your cake and eat it too:-) Training induces adaptations specific to the type of training stimulus. The training stimulus activates a series of signaling pathways that up-regulate certain gene expressions. The principle pathway caused by your running is known as the AMPK that up-regulates mitochondrial biogenesis leading to better endurance in those trained muscle fibers. Heavy strength training induces Hypertrophy through the mTOR pathway.
The conflict comes because the AMPK (endurance pathway) also inhibits protein synthesis (growing new muscle fibers). You’ll loose muscle mass, especially fast twitch (high strength) muscle fibers when you run high volumes.
The mTOR pathway (Strength gains through hypertrophy) will cause you to be carrying more muscle around which will reduce your running economy and endurance performance. Some studies have shown that mTOR also down regulates the AMPK pathway.
The upshot of this is that strength training may not directly hurt your endurance but adding muscle mass will reduce your running performance. Endurance training will definitely have a negative effect on your strength. It is a trade off and only you can decide how much of a trade off you can tolerate.
I’d recommend you focus on max strength (4-5 sets of 2-4 reps with 90% 1RM) methods rather than hypertrophy (6-8 sets of 8-12 reps going to failure in each set) methods to maintain and build strength with minimal mass gains.
I hope this helps.