Hello. I will be in Paris for the first time for two days at the end of August and I am staying at the Hotel Mercure Austerlitz Bibliotheque. I have read about how "walkable" Paris is, and the quality of the public transit, but I am wondering about travel times within the city. For example, how much time would it take me to get to the opposite end of the city where the Champs-Elysees and other such tourist attractions are using public transit? I want to leave as much time as I can for exploring, but there are also some sites that I really want to see.
Thanks for any advice!

Well, it looks like your hotel is right by a metro stop (Bercy) on the 14 line. You can take that straight to the Louvre, for example. The trains on the 14 line run fast and frequently.
Your hotel is actually right beside "Quai de la Gare" Metro station (line 6), but do as #1 suggests. Walk across the bridge to Bercy (line 14). Using the Metro, you can be anywhere in central Paris within 20 mins.
Just plan a route, take the metro to one end of your route, then walk.
I agree with the 20 minute figure. 30 minutes max for the most remote areas of Paris. (Add 10 minutes maybe after 10 pm, for waiting time.)

The Metro is v efficient and the trains are regular and don't forget the buses - they are really worth checking out - might take you longer to get from a-b but you get to see the city en route! And gives you a sense of where everything is. And besides no matter where you are in paris there is so much to see and do - stop stressing about how long it takes to get aroundd, just relax and walk and get a feel for the place - it is worth it.