The code 1M distribution is what paid off the loan. Effectively, a distribution was made from your 401(k), reducing the balance in your 401(k), that was turned around to immediately pay off the loan. IF you can come up with the money to put the offset-distribution amount back into a retirement account like an IRA, you avoid tax and penalty on the offset distribution and you report the code 1M Form 1099-R on your tax return as having been rolled over. If you don't come up with the money to complete the rollover, the code 1M distribution is subject to ordinary income tax and 10% early-distribution penalty.
You have until the due date of your 2018 tax return, including extensions, to come up with the money and complete the rollover. When you said that you used your own funds to pay off the loan, I assumed in my answer that you had actually done this rollover (since the code 1M distribution is what actually paid off the loan).
