You don't need to use an intermediate party to use bitcoin. You can submit transactions directly to the network from your wallet software without running a miner. The miners secure the network, but the nature of the system is that they are trustless, so long as %51 of them are honest. By having the cost of mining being high, that makes the cost of an attack on the network high. Which aligns the miners incentives (Eg: keep the price of bitcoins from going to zero as it would if the network were compromised) aligned with yours.

The blockchain is a bit large at 80GB but you don't need to download the whole blockchain to run a wallet and make transactions there are "light wallets" that will do this, and currently in testing is an even more efficient method of doing this called segregated witness.