Running long distance is probably the second best way (after swimming) to have a great cardiovascular health.
Quickest way to improve your time, form and reduce injury is to combine running with some strength training. See Weight Training for Running and Core Stability for Runners.
For the broader philosophy of staying close to race shape year-round rather than peaking for a specific event, see Lifestyle Running. For a structured catalogue of treadmill-based workouts, see Treadmill Workout Types and Marathon Goal Pace Zones.