I did complete the City to Surf run/walk/crawl a while ago, but couldn't walk for a couple of days afterwards. Admittedly, I didn't do much training for it, but I figured since I can walk around the shops for several hours, I should be able to walk from the city to Bondi. Ouch.
I like to do a couple of fun runs, mainly because I enjoy running with other people and being around other people that are motivated to be healthy and active. I do get put off by the cost of some fun runs (especially high cost runs that are not linked to a charity) which is why parkrun is my favourite regular fun run. It is a free, timed 5km run that is held weekly in various locations.