This command will put the boots on your feet. Be careful! They can cause lag. In adventure/survival mode they make you invincible.
The wither can't hurt you at all. Instead, after five or six shots at you, it dies due to thorns. Also, one punch from you can kill an iron golem.
If you combine them with elytra, you can get a running jump and start flying. You can get above the build limit like that.
You go even faster in water, so be careful! It is easy to get lost. Also, try opening chests in dungeons, villages, temples, etc.
NOTE: You need to do "/effect @p instant_health 1000000 100 true" to make the invincible thing work.