I go through the same thing. If you are doing serious cardio and not just old lady walking on a treadmill your squats will go down. I think you just have to accept the endurance gain vs max strength drop. Max strength is a worth while pursuit but without true conditioning it's almost worthless in most athletic activities.
Last year I decided to run in a local 5K race. I started running 5 miles at a clip and my lifting really took a nose drive. I think you have to compromise in both areas. You won't be at your max at lifting or running but you will be good at both.
Presently I just made the same mistake as last year. I went over board on running and I burned muscle off at an alarming rate. I think this year I'm going to keep my long run at 4 miles and include a lot of short ones from 3 to 1 miles. Maybe some intervals.
Keep protein intake high. Drink a lot of water because it's easy to get dehydrated when doing a lot of endurance work. Make sure you have rest days and some easy days with cardio for recovery.
What's you next career? It sounds like law enforcement, fire, or military. In my career we have to run the 1.5 mile every year or we can't be considered for promotion. In basic I ran it in the low 8 minute range and now if I break 10 minutes I'm happy.
The best way to preserve muscle strength is to do the majority of your cardio work doing intervals. Something like 6 x 400 meter repeats, 4 x 800 meters or 8 x 200 meters will keep a strength factor involved in you running and it will still give you that 1.5 mile you require.
When Roger Bannister broke the 4 minute mile barrier all milers ran only distance during training. Bannister ran almost exclusively 10 x 440 yard repeats. This was considered radical training back then.
