Layover in Munich
Remember that your 20 hours actually gets reduced tremendously when you figure in the time it takes upon your arrival into the airport: depending on where you arrive from, you may have to go through passport control/customs. Also, remember to check to see if your luggage (if you check) would continue through to your next destination or if you will have to claim it. You may need to check with the airline on that. Claiming baggage takes time as well. Figure about 90-120 minutes for this process.
Plus, consider the time it takes to return to the airport from the city, depending on your next destination, you may have to arrive 1-4 hours early, plus enter through security.
Also, depending on the service you take, your travel time into the city can take time as well, approximately 45-60 minutes (one way). Here is the MUC's website for your options for public transport to (from) airport: http://www.munich-airport.de/en/consumer/anab/index.jsp
Adding all of these factors, your what-was 20 hours doesn't leave for much time to see the city. You won't really have time to "see the sights" so to speak, but you may be able to take walks around Marienplatz (City Center) or other nearby areas.
A 20 hour layover would not be fun. Be extremely cognizant of your time limitations should you decide to do so. Myself, I would be too scared that something would happen, so I would probably opt for a hotel room at the airport hotel.