Realistic Vanilla TNT Run

Solution 1:

type this into chat:

/scoreboard objectives add time50 dummy

Then put the following into unconditional command blocks:

/scoreboard players add @a time50 1

execute @a[score_time50_min=5] ~ ~ ~ testforblock ~ ~-1 ~ stone

This one goes in a conditional command block:

execute @a[score_time50_min=5] ~ ~ ~ setblock ~ ~-1 ~ air

This one in an unconditional:

/scoreboard players reset @a[score_time50_min=5] time50