If Marcy can walk 12 blocks in 5 minutes, then she should be able to walk 72 blocks in 30 minutes.
See how we went from 5 minutes to 30 minutes? We had to * 6 to do that, so we did 12 * 6 to get 72 blocks.
If each block is about 50 feet long, then 72 blocks would be about 50 * 72 = 3600 feet.
Answer: About 3600 feet