Well, according to the people in the running shop I've just come back from, the answer to the original question is definitely NOT today, when you plan to run the FLM in them.
They'd just sold a brand new pair to someone who was doing just that - and wouldn't listen to reason.
I've just bought myself a second pair. I've been wanting to for a couple of weeks because I was fed up of cleaning every last bit of mud off them after my outdoor run before using them in the gym. It seemed like an extravagance though, but now that I've read (elsewhere actually, but nice to see it confirmed here too

) that there are good reasons for alternating, I decided to go for it. I'm not sure that with the distances I'm running it is that justified, mind you, but I've done it now
