I am in Marathon training at the moment - 5 weeks today!
My current speed is a mile every 10 mins - so I hope to do the marathon is under 4 1/2 hours - it will be really tough but hope to do it :)
My fastest time for 5k is 24 mins - but this was in a 'race' type environment - I couldn't do that today.
I am 37 years old, and overweight (but getting slimmer!).
Not sure where you live, but find out if there is a ParkRun near you -
http://parkrun.com/home.aspx - this is a great way of exercising - there are people there who walk it and people who will do it in around 15 mins!
If you want to run by yourself and increase your time, consider using an App like RunKeeper if you have a smartphone, or looking at a Garmin GPS watch - this will help you get faster times.