What is the furthest distance cycled in one day?

There's really no sensible answer to that, there are too many variables:

• calendar day(24 hours) or only daylight hours
• solo, or group ride
• road, track or off-road
• flat, hilly or mountainous course
• carrying stuff or stripped for racing

Just to mention a few.

A strong group of road riders can maintain a rolling average of 40 KMH/25 MPH for 6-8 hours on a fairly flat course, so probably around 800 km/500 miles in 24 hours.