Minecraft – 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.