hello janbei,
welcome to the club of oldies who have decided to do the
camino de santiago.
i am an oldie like you with no experience of hiking, short or long distance. i was 2 months shy of 70 when i decided to do my second camino, the via de la plata (camino mozarabe), alone during this past february/march from sevilla to sdc in 36 walking days, a distance of 1000 kilometers (including getting lost). this excluded a few rest days in merida and salamanca. my first camino was the
camino frances in may/june of last year from sjpdp to sdc in 31 days, a distance of almost 800 kilometers to included distance getting lost.
based on my experience i would like to offer a few suggestions. you will be starting in april of next year. first of all if you are fairly fit and can walk 15-20 kms each day, you should be ok. the camino is not a marathon and this is your camino. you can walk as short or as long a distance as you like. you are not cheating by having luggage transported or taking public transportation at some sections. if you aim is to obtain a compostela at the end of your camino in sdc, you must walk the last 100 kms on foot.
keep your backpack as light as possible, preferably 10% of your body weight not counting any water or food. there are long stretches of stage without any access to water fountain. i don't know what the weather is going to be like next april. if it is any indication like this year, you might want to make sure you are warm. all the municipal and xunta albergues will be opened from april on. don't overload your backpack. in some sections you will need to bring 2 litres of water (equivalent to 2 extra kilos). you will more likely encountered heavy rain starting from zamora up the galicia coast to sdc. so make sure your contents in your backpack are protected with plastic bags or ziplocks. use a rucksack cover or black rubbish bag to protect it from rain. also good idea to have rain gears. my preference is for a lightweight poncho rather than a waterproof polyester rain jacket. start slowly. you will notice the first stage is quite short. i don't know if you are a history buff or not. if so, you might enjoy italica, caparra, merida, caceres. take rest days in merida, salamanca and ourense (thermal baths).
also listen to all the advice from alan & company regarding what sections to bypass with public transport and also what to see.
let me know if you need any more info. buen camino and god bless.