The ubuntu d'load must have been a goodie !
Using ubuntu mate from the usb is called running it LIVE
If you were to click on "install" on the desktop, that would WIPE win 10 and install ubuntu on your hard drive......it would be the only OS (operating system) on there....windows 10 would be gone...kapoof !
A dual boot means to have BOTH windows 10 and Ubuntu on the same hard drive.....you can choose which one to run form a thing called a grub menu which shows at startup time
I would leave that for now. Have a play with ubuntu mate....i have not used it, but have been told it is good to use.
Have a serious play with it......you cannot do any harm.....if you do screw it up...all you have to do is reboot....back into ubuntu...and away you go again.
Keep in mind that any changes you make to ubuntu while it is running on the usb...such as setting up Firefox or Thunderbird email or whatever....will NOT be there after you have rebooted or taken the usb stick out. It will return to a stock standard ubuntu mate with no changes.
The ease which you obviously experienced in booting up ubuntu mate is how it should always happen......easy as pie.
See how you like ubuntu mate
Then repeat your success with Linux Mint
I smiled when I read of your success.....and I am sure wiz will too !