I just went on the first bit of this route. Unfortunately, due to some achilles trouble I can't run right now, but I walked from T1 over into the woods using totti's suggested route for around 6k total. And I'm looking forward to doing a real run next time I have a substantial FRA layover.

Fwiw, I found the luggage storage service between A and B baggage claim to be quite convenient. It felt a bit pricey at first (4.5 Euros for my rollaboard for <2 hours) but when I compared it to the overall cost of my flights and the enjoyment I get from getting outside it was well worth it.

One other thing: if you have access to the LH arrivals lounge, I think that's a great place to shower & get food. (I've never been to FCT so I can't comment on that, but obviously it would be better if you do have access).

Thank you again for providing the information about how to do this!