I have said here many times before that, although C25K is a brilliant programme, it may indeed be too much for some people at first. So a little pre-course preparation is not only appropriate , but will even do you good. I started my journey 5 years ago at age 67 - at that time I could see that C25K was too much for me - so I started out on a walking programme for a month . I can't even remember what I did now but there are lots of programmes available like this one blog.myfitnesspal.com/4-wee... I then did this very easy introductory running programme from the famous MAYO clinic. It does not aim to run continuously but I did get out to 4 miles by the end. mayoclinic.org/healthy-life... This plan introduces you to "run/walking". This all took me about 3 months before I started C25K which I then completed in the set time and could run non-stop for 30 minutes . It took me quite a long time before I finally was able to run a 5K parkrun in under 30 minutes 
. 5 years later and I have now completed 3 half marathons and am now training to run/walk my first full marathon at the ripe old age of 72
For most people, 5klms IS a marathon - so start your marathon training right now, but slowly and easily
I was born in 1946 too, and just doing C25k for the third time. Walking did it for me as well, I can walk a Half Marathon in about 2.25 and a 10k in 65 minutes, but doubt if I could run those distances at the moment, so like so many here, you are an inspiration.
You must be a fast walker!!
- I run/walk those times for the 10K and HM 
I am an 'also ran' in most races. Some of those I walk against can do a 10k in 42 minutes . Mind you they have a fabulous technique and most of them are 40 years younger The ambition of all of us more mature walkers is to break the hour, but my farmers lope technique will never get me there. But we are still going and giving it ou all, which is the important thing.