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