All the times I've been lucky enough to be a part of a show that's actually gotten on the air, it's always that same mixture of excitement and utter fear.
I try to avoid the temptation with time as a total indicator for what my possibilities are for the marathon. It's the not the best indicator, but it's more how you feel, how you cover the distance and how you are able to do the training afterward.
Here's good advice for practice: go into partnership with nature; she does more than half the work and asks none of the fee.