Running a marathon is hard - really hard. Most people need month to train for a marathon and only run maybe 70% of the distance during training. A designated city marathon offers a lot of benefits that you don't get when running alone:
- Blocked streets, no need to wait for traffic lights etc.
- They are at a specific date in the future which helps
you by setting a deadline to get in shape
- Spectators to cheer you on
- No matter how fast/slow you are, there are hundreds of
people around you, running at the same speed
I don't know about you, but even people who run regularly are physically not able to run a marathon tomorrow or next week. So how many people are in your area who are able to run a marathon and are just as fast as you are?
OTOH: What about a site to set up "running groups". You specify a track/expected time and how often and when the group is running (monthly, twice a week etc.). This way you give people a way to organize groups, let them communicate if they are unable to show up, talk outside of a running context etc.
You might want to look into similar sites. Off the top of my head I know of runmap.net and mapmyrun.com but there are probably a lot more like this.