Don't forget that unless something's changed the strength of the Marine Corps is established by public law, so there's a limit to how far you can shrink it before you need to change laws.

Since the Marines have their own organic air (fixed wing and helicopters) they can respond to a variety of situations quickly and with a good combined arms mix. That's just a given that isn't going to change unless the AF chops close air support assets on a permanent basis (or just gives them up like McPeak was talking about in the early 1990s) to the Army. The size and culture of the force also allows them to reorient faster than the Army when it comes to changes in doctrine and operational methods. They can't sustain some training facilities (hence the guys at Knox and elsewhere), but that has to do with budgeting.

The Army is better-suited for prolonged engagements/deployments due to both their size and logistics capability. That's pretty much a given as well. You need both the Army and/or Marines to secure bases for the AF, since operating out of theater isn't always possible or practical (nations can restrict overflight, after all, and tanker ops are limited by numbers available).

Could the Army be trained to do some Marine activities? Of course, but by the same token the Corps could expand and take on many Army duties as well. By nature, history, and training the Corps is an expeditionary force. The Army is not to the same degree. Elements of it used to be, but the great "reforms" of Root and his later followers really changed that. You do see sparks of it from time to time (mainly in the Cav community from what I've seen, but again my experience is somewhat limited when it comes to the infantry side).

And Stan, the AF's been allowing jogging for some time. All you have to do is complete your 1.5 miles in the time allowed (up to 14 minutes or so if memory serves).