How much running should I start with?

Hi everyone, I need some good sound advice, especially from any running coaches or runners who have been through similar circumstances. I ran Chester 3 weeks ago in a time of 3:12. Going into the race I had a couple of niggles and I guess running a marathon turned at least one of them into a fully blown injury. I feel like I might be able to attempt running again from Monday as I've not been able to for the 3 weeks, but I have no idea how much running I can get away with after not running for 3 weeks? Can anyone advise me? Many thanks

You won't have really lost any fitness in 3 weeks, so from that point of view you could go out and probably run anything you want!

However, the real issue is your injury/injuries and whether or not they have healed.