It's very easy, we've done it before with kids & DH does it every few weeks for work. Are you sure it's terminal D though that the Marriott goes into - he thinks it's B or C. He said it's a USAir hub terminal that it connects into, because he only flies USAir & never has to go to another terminal from the Marriott. He said it doesn't matter - whatever terminal you walk into from the parking deck of the Marriott, you go through security there & then you can connect to the other terminals.
We/he always just exit out the back of the hotel & walk through the parking deck to the terminal - you mentioned a walkway - maybe that's another entrance different from what I'm talking about. Either way, it's easy. The walk is maybe 3 min. tops (out the back way). It's under cover but still outdoors so cold if it's winter.