Training for the first parkrun day 
It's highly unlikely and even dangerous to go from the couch to run a 5km without some degree of 'training' especially........ dare I say it! at your age! So if your body is ok and the advice from the GP is good-to-go....move to the next stage, get the right kit and do some pre-parkrun training! Remember...........I started my parkrun by just walking, then quick -walking , followed by interval-shuffling before finally........steady jogging. So building-up quite slowly really works and chances of injury are fewer. My biggest problem was lung-capacity, or lack of it. Even always being a non-smoker to keep going for, in my case, 40 + minutes, it was tough. Don't panic as It should soon build up , which can never be a bad thing for you in your later life, or indeed, anyone! Plenty of clubs are able to offer a couch-to-5ks training schedule that can be taken over say 6, or more likely, 9 weeks, followed by a local 'inaugural' parkrun. Just check out your local running club or go on-line. It's a great way to do it, with like minded people and experts giving you knowledge and support and turning up to your first parkrun with yournew found chums can be much easier than facing it all by yourself.
As you 'mature' you never know when you may need your new found fitness ; perhaps trying to keep up with the grandchildren or maybe running for the last bus.......so it's a sure bet it will be come in really useful!
