Difficult and plain boring to do in the UK but should be easy enough across Europe. Forty years ago I was doing 1000 miles days across Europe on a Honda CX 500.
but why would you want to do it ?
i've done 1 x 1250 mile day , most of the day was ok , but the last 5-6 hours were a real struggle .but the day ended up as 22hrs on the bike. not planning on repeating the experience again.
Not sure I guess to set a record or to just be able to say you did it? Some climb up mountains. I wouldn't be capable of doing that nor would I want to if I could, but it makes for interesting discussion.