Hi c-g_24,
For most people 50 miles a day is very doable indeed - that's only 10 miles more than my commute (and I'm 43)! However, I do agree with people that you should factor in a couple of days off - no matter whether you're doing 10 or 100 miles a day. Sometimes you want to stop somewhere, sometimes things just happen. Last time I was cycling in the Perpignan area, for instance, I got stranded near Narbonne for two days because the wind was too strong to cycle in. Oh, yes, the south of France isn't all sun and fun - the winds can be ferocious. You might have heard of the Mistral? Well, that's only an issue in the Marseille/Provence/Rhône valley area. The Mistral is very strong - but at least you know what direction it's coming from. But in the south west you get winds coming from all directions and at any time. It's probably less of an issue in April than in the summer months however.
Is there any reason why you want to start in Perpignan? In my opinion it would be easier to do it the other way round. The hills in the centre of France can be very tiring. If you start in Caen it'd probably give you time to build up your strength and stamina before you meet them. On the subject of planning, most roads in France are a pleasure to cycle on. But remember to avoid the N roads; and although most of them are fine, be careful even of D roads because they occasionally become practical motorways.
I second the use of Via Michelin - it will help find cycle-friendly routes. I would also use ridewithgps which has a very handy gradient profile feature that will help you with the hills. With ridewithgps you can download your route for your gps (I use osmand - which is free and can be used offline - very handy if there are roaming charges).