I've always parked at the shops at Georgetown off M. It can be packed before the race starts. Not sure how the 5k early start will impact it this year.
There are a bunch of portapotties down near the start line. There are lines for them prior to the race start (busier the closer it gets), but once runners are on the course, should be no problem.
There's a starbucks on M street near the start that I think is usually open before the race. But I'm not sure if it is open before the 7 am 5k start.
In my experience, this is one of the biggest races in town, so be prepared for the crowds.