10lb of honey in 6 gal of water should get you about 1.058 OG, 20lb cranks you on up to 1.115. Either is technically OK for a mead, but 20 bumps the gravity way up and you should probably pitch at least twice as much yeast. Ending ABV will go up correspondingly with OG.
Remember 20lbs of honey has significant volume so you will need to adjust the volume of water/honey to fit your fermentor and still leave reasonable head space.
Temp depends upon the yeast strain you use. For example, EC-1118 has a wide temp range, but 15-25C is optimal. So at about 70F you should be fine. Higher temps are more likely to produce off flavors. Check the specs on the yeast strain you chose.
Be sure and read some of the stickys/wikis/etc on the mead forums here, especially those by "hightest". Some great info there. A key difference between brewing meads and beers is aeration and nutrient additions. Nothing particular difficult but be sure and read up on these subjects. Also mead fermentations tend to be longer running.
Personally, I like to go with basic traditional meads and then blend to make melomels and braggots. This approach gives you a lot of flexibility and the ability to blend flavors to your liking. For example, if the flavors of that big melomel you just made are just too overpowering then you can tone it down a bit by blending with a traditional mead. And, you end up with more interesting stuff to drink.