Middle Distance Weights and Speed

Do the slow long runs need to be isolated pre season? Or can it be incorporated into the program, replacing tempo?

Because I have heard from athletes that too much aerobic base running will dull their speed. Would training these two qualities concurrently conflict?

The particular athlete I have in mind runs as follows:

Personal Bests:

100m: 11.0s
200m: 22.3s
400: 48.9s
800: 1:47
1000m: 2:24
1500m: 3:49