Hiya from the South of England.. There are only rolling hills and flat bits down here, the Mountains are concentrated around the North East and in Wales or Scotland..
The whole trip depends on your fitness / timescale for travel / accommodation / terrain. Every one is different, some prefer to crack on, others to wander / take pictures. I am capable of doing 16 - 20 miles in a day but a friend of mine can do that distance going over mountains.. if you are used to lowland our fitness for hills / mountains will be poor so be aware that a holiday walking in the mountains will be slow and hard going. We too love going to the mountains but because we live in such a different terrain we tend to stick to 8 - 10 mile day trips.. I am 47 and have been walking for 12 years, I have only done 2 overnight stay type trips, one in Wales (Snowdonia) and one locally in Hampshire for 2 nights per trip. ( I'm not going to count a motorcycling camping trip I did when I was 28)..
Any walk is possible, just depends how prepared you are.. As you will be aware the daylight hours are short so how about booking a couple of Bed and Breakfast on a short-ish walk and enjoy a good beer and save the longer more challenging walks for warmer weather when the days are longer ?