Which is uncertain

At the beginning it is always a bit of a struggle. At the beginning it just doesn't feel good. After the first hundred meters incipient sweat is already leaking through the pores of my skin. The unpleasant effort lasts until running turns into a fully aerobic exercise. That occurs around the third kilometer. Then, a significant thing happens - after years of training my body is so much used to be in running mode that running becomes suddenly as effortless as walking. My body behaves like a quantum-mechanical system, a linear combination of three independent states, rest, walk, and run. I am long acquainted with the first two, and slowly making friends with the third one. Training holds the key, there is little secret. The goal is plain and simple, to stretch the period of effortless running - when it does just feel like walking - for as long as possible.

Running effortless at a 5 min/km pace for as long as possible.

For 42 km and a couple hundred more meters.

This is the goal.

Which is uncertain.