Yegods, if you are running a bit and jogging and walking you are probably doing it in under 45 mins. Many people are like this at the beginning, especially if you have any hills on your park run course.
We also have people who are recovering from illness or surgery , have lost a lot of weight or getting over depression or a bereavement . So many people have had to over come major obstacles to just be able to walk /jog 3 miles. They are not just welcome at park run, they are one of the best things about it!
There are so many inspiring stories. And these are always the people who are so appreciative of the volunteers and so apologetic about being " slow".
Poor young lad with poorly controlled asthma was about 11 or 12. So old enough to do park run on his own. But his parents have now been asked to supervise him due to medical issues . The volunteers are just ordinary park runners, they are not paramedics and can't be given such a responsibility.
For the avoidance of doubt, children in general are very welcome at park run, although younger ones need to be supervised by a parent /adult. My 9yo is now faster than me so I need to get one of my friends to run with him 
If you are worried about being last, you can check out the " latest results " page for your local parkun. The second last time on the list is the slowest person. Look back a few weeks and you will see it's probably about 45-50 mins