How to estimate how long a tour will take? (UK to Greece) If someone of moderate cycling abilities, and pretty good fitness tried to cycle to Greece from eg. London; how long would it take approximately. Assuming around 8 hours of cycling a day?

And please (for future users) explain how you did the calculation and link to any on-line tools or resources you used.
It's more or less a math question. A way from Athen to London without using a ferry if not absolutely necessary (Canal), it is approximately 3200 kilometers.

So if you have an average speed of 15 km/h (mountainbike) or maybe 25 km/h (racing bicycle) it would take approximately 214 respectively 128 hours to cycle the route.

As always for such rough approximation I would add at least 20% time buffer. So you would have 257 hours (32 days) respectively 154 hours (19 days).

I also generated a tour profile for you:



Here you can see that the route is more or less flat, expect for one or two passes that you would have to climb. But be careful, on such a large scale, all steep climbs aren't visible. The red line indicates Zagreb (Croatia).

The largest mountains on the road are between Graz and Wels (Austria). But with a few extra kilometer you could avoid them when traveling over Vienna.

Then also the evalation profile looks much nicer (unfortunately I can't draw it in the same scale):



Here the first red line is Zagreb and the second Vienna (Austria).

Traveling over Budapest (Hungary) would be even more flat, but now the distance is really longer:



Here the first red line is Zagreb (Croatia) and the second one is Budapest (Hungary).

In order to draw all the profiles, I used GPS Visualizer. It is a online tool, that can plot profiles from various sources (e.g. GPS tracks). In this case I just created a tour in Google Maps and then used the static link to this tour as input for the GPS visualizer.

Just for the fun of it, I created you a very alpine route:



The first red line is Brindisi (Italy), the second is Vaduz (Liechtenstein) and the third is Sion (Switzerland).