Hi there!
First of all, Blinq, you are living in Holland but not able to ride a bike? tut tut...
Vak,
It is hard to give you an exact answer for the 60 miles because only you know how much your body can handle but I think it is going to be quite heavy. My father did the route Holland to Santiago de Compostela, biked different distances every day depending on hills / mountains / weather etc. I remember that he mentioned one day where they biked a little over 70 miles and he was beaten, and he is used to biking 25 miles a day for his work... So, it is doable but you will probably have some heavy days.
Keep in mind that on a straight road you will bike about 12 miles an hour, especially if you also want to see something of the surroundings, on a mountain road this is going to be less especially if you are not really used to it. Also keep in mind that you need some days to take a rest for at least half a day otherwise you are going to be completely broken I am afraid.
As for the locations: why don´t you take the tunnel train to France start there with a route? It is a beautiful country to do some cycling in, hills, surroundings, lovely small villages etc. You could take (a part of) the St. James route (to Santiago de Compostela) it is supposed to be very special, even a bit spiritual (don´t know if you are into that). Another tip is to go to Spain, like France a perfect country for biking. I would not go to far off from England if you have to be back in three weeks and would just bike one way and take plane or train back.
http://www.bikespain.info/ - bike routes in Spain
http://www.biketoursfrance.com/ - bike routes in France
OHannemarie
(Oh-Holidays)