Hi Lizzie,
Welcome to the forum. I remember when we first began planning, we were just as nervous as you are. But part of the fun and preparation is checking out everything this forum has to offer. We even used Google Earth to get a 'feel' of the road and path conditions. I agree with Unadara above that the
Camino Frances would probably be the best one to begin with. Although one of the longest, and it goes over three huge ranges of mountains (if you start at St John Pied de Port), it is also one of the best as there is always something to see, small albergues/hostels dotted along the way, and plenty of "pit-stops".
We did not have any trouble with finding small double rooms along the way, although we did also use dorms at times as the beds are cheaper. We paid, on average, 35 or 40 euros for a double room, as opposed to 5, 10 or 11 euros each for a bed in a large dormitory. It might also be worth noting that there is always a local bus you can catch between villages/towns. Above all, remember that the Camino is meant to be a wonderful experience for everyone and it's not how many miles you achieve, but what you get from doing them.
If you want the "Compostela" (ours proudly adorn the wall in our front room) when you finally reach Santiago, it helps to know that only the last 100 miles (if on foot, or 200 if on bike) need to be recorded in your Pilgrim's Passport. Many people do the whole stretch from St John Pied de Port to Santiago; some begin in Roncesvalle, or even Pamplona; while most only do the last 100 miles. To be honest, I did not feel we spent enough time doing the route by bike as the three weeks sped by far too fast. It also gets easier as you go along - I have heard that many people give up after only the first day or two, which is a pity because if we had done this, I know we would have bitterly regretted not sticking with it.
I suspect the Meseta might be challenging from a walking pilgrim's point of view as, in places, it is very long and flat. But even this can be turned to your advantage and help you to "recharge your batteries" before you hit the hills again. We rode the route and met people who said they wished they were riding, while I was wishing I was walking! As a group, you will have an amazing time, and a wonderful experience. You will never regret the decision to do the Camino. As an afterthought, I am 65, and my husband is 72, and we are planning another return next year as well, although the consensus as to whether to use the bikes again, or get out there on foot is far from resolved!