[Felix The Assassin]

3 miles after leg work?
Push-ups after arms?

Three areas of concern for your legs. Hamstring, ACL (knee), and achiles tendon.

Ensure you are warm and properly stretched prior to running, more so if you have done heavy work, squats for example.
Knees, think of them as the second shock absorber, first being a high quality shoe.
Achiles is the tendon that connects back of ankle to calf area.
Hamstring, easily pulled if not stretched or warm, especially after a heavy lift.

Push-ups, no bigge! I use to burn my unit on Tuesdays. 150 push-ups for warm up (50 per set), then muscle failure(around another 300), then .50 cal barrel (25.lbs) curls, another 100 in four sets of 25, then 15 count pyrmid, then another two sets of 50.
Some reason they never complained about wearing a 25+ lb vest!