Hamstring Strain

Hi there! Sorry about the hammy! Those are painful and can be difficult to come back from. You are definitely doing the right thing with the ice and comp and meds and rest. I wouldn't run on it for a few days--not even until the 20-miler (curious: what mary are you doing? I also have a 20-miler this weekend). Depending on how your muscles feel, you may not want to do the 20-miler, or cut it short, or replace it with water running (boring, but it will definitely do the job fine), or do a half-land half-water. You have a lot of options!

Most of the sources I've seen advise AGAINST stretching a strain until it's healed, unless it's ITBS.

The following advice is paraphrased from Jeff Galloway, who ran his marathon PR eight weeks out of a hamstring strain (p. 120, Marathon): 1. Shorten your stride. It reduces the tension on the hammy and allows healing even if you are running. 2. Slow your pace. 3. Nearer the marathon, IF you do speedwork (don't start now if you don't!!!), do super-short-stride hill repeats.

I'd give it a few days before you get on a bike (in other words, not 'til after the 20-miler or whatever you decide to do) and then take it easy at first to see how it feels. Swimming right away should be just fine, but I wouldn't do explosive push-offs from the wall or anything like that. 

Good luck!