I haven't been there myself, but Big Wheel Bikes has rentals, http://www.bigwheelbikes.com/

For training routes check out: http://www.bikewashington.org

Depending where you are staying, you could do speed workout at Hains Point-right downtown(there are group rides Tue/Thurs 6:30AM or at lunch time, check out: www.dctriclub.net/forum), long ride on Saturday leaving from Capitol Hill Bikes at 8AM http://capitolhillbikes.com/index.cfm.

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“You can't coach desire.” --Dathan Ritzenhein
http://xtreme4.com/ -- an extreme ride for extreme change: go green

Big Wheel rents bikes, but I don't think they're going to rent what you had in mind. They're a tri/road/comfort bike shop, and guess which one they do hourly rentals of...

I'd message the DC Velo guys, or look up NCVC and ask them what your best options are.

For training, your best bets are:
Hains Point (the only truly flat spot around here) with lunchtime rides leaving at about 12:15. Some days are definitely hammerfests. It's down near the Jefferson Memorial.

Mt. Vernon Trail - avoid like the plague on weekends, good for mid-week rides. Key Bridge to Mt Vernon and back is approximately 40 miles.

Capitol Crescent Trail - much like Mt Vernon, but it goes the other way. Again, not weekend-friendly.

A good website for road-based hints is http://www.babesonbikes.org/
I know it's a girlie club, but they've got the best set of cues online. It's pretty well organized, and you should be able to do some of their training rides without crazy difficultly finding the roads.

Good luck!