You can absolutely ride long distances in clipless pedals.
There are countless examples of this - at the extreme, look at any picture of a Race Across America participant, there is a very strong chance are they will be using clipless pedals of some kind (RAAM being a 4800km race, completed solo in less than 8 days by the winners..)
Clipless pedals are essentially ubiquitous in modern cycling, from "recreational" level up to madness like the aforementioned Race Across America.
However, it is vitally important to have the bike properly setup for you ("bike fit"), a major part of this being the pedals+cleats. Like all adjustments, you should be careful to slowly adapt to the new setup (i.e don't attaching the pedals and going straight out on a 1000km ride!)
I am informed that I can increase speed wearing clipless pedals
This part is more debatable - clipless pedals don't necessarily make you faster, but they do have other benefits - e.g keeping the feet in the ideal position (potentially avoiding knee injuries etc), preventing your feet from slipping off the pedals (e.g in the rain, going over a pothole)