fast walk, builds a bit more muscle, muscle uses fat for energy 24/7, makes you lose fat.

Does not matter.
Work done (energy used) = force x distance moved.
The force used is to move your body mass, so it stays much the same.
Rate of doing work, Watts = work done / time taken.
Overall then a short, quick walk could use the same energy as a longer, slower one.
What you must consider is whether you are fit enough to do fast walking. If you are still too fat and unfit then a slower but longer walk is safer to start with.

well both ways are good, but fast walking will give you cramps. i suggest a medium pace for an hour, you wont get cramps that way.

medium pace for an hour

I think the best thing to do is mix it up in intervals. Walk fast for 5 minutes and slower for the next five. Keep on increasing your speed as the time goes on. Do this for 30 minutes and you will actually burn more fat than if you were walking fast for 30 minutes straight.

fast walk for an hour :D
lol.. nah, spacing stuff out, is alot better for you.. this include food, eating 6 small meals, if alot better then eating 3 big meals, when trying to lose weight.

I agree with caligirl mix it up. Everyone is different but that works for me. Try the below link for more specifics.

Med pace for 1 hr.

walking at a medium pace for one hour. This is great for fat burning. If you carry light weights when you do it you can really burn a lot of extra calories.