Paraphrase this:

muscular endurance is the ability of the muscles to perform continuous without muscular endurance is the ability of the muscles to perform continuous without fatiguing.

Having a high amount of muscular endurance allows the body to use it's muscles for an extended period of time without tiring. Hope this helps :)

the gravitational force on an object increase as the mass of an object also, im not going to report you, but this question should be under the science category, not health

you may having too much sleep.. a good night sleep consists of about 7 hours. more than that could be the reason you're still tired throughout the day.