Day 7 and I can't run as much now...does it come back later in the fast?

So far things have gone well, and I found that the fast improved my endurance in running, but now it's gone the opposite way. I'm planning on going for 30 days...does the strength come back at some point?

I'm in a training group for running a 10K and I'm concerned that I won't be able to keep up while I'm on this fast.