How to make a block disappear after a certain period of time

So I am making a 'magic trail', which is essentially wherever you walk and jump, a block will be placed below you. Unfortunately, the trail will always stick around and never go away. After a while this means that there will be a giant trail of blocks through the sky that looks pretty ugly. What I want to do is make a block disappear after a certain period of time, say, 7 seconds. Here are the commands and the setup for this project:

Command block #1: /setblock ~ ~ ~-1 redstone_block 0 destroy
Command block #2: /execute @a ~ ~ ~ /fill ~ ~-1 ~ ~ ~-1 ~ wool 0 replace  air
Redstone block: []

            #2
Setup:   #1 []

Note: Please don't try a method using a FallingSandEntity and Age because I want to have the player be able to actually walk on the blocks. If you use a FallingSandEntity the player will fall right through the block. Anyways, all help is appreciated, thanks! Also, if you want to try doing this with the fill command, I would want a 7x7x7 hollow cube surrounding the player so that the trail can't go outside the blocks. I just don't know how to do this.

I did something very similar to this, using a fill clock, scoreboard commands, and wither skulls.

Two scoreboard objectives were added, walkonair and lifetime. The first objective controls the ability to walk on air, and the second objective controls the "lifetime" of the blocks below the player.

The setup: