While I'm no lawyer, I would say you don't have to provide a physical disk if you are providing a download.
People may want to refer to
particularly section 3b
"Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange;"
The way I interpret "physically performing" can include offering a download link.
There are a few sections like
"If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code."
That sound like you have to provide access to source code in the same medium as the program, but it sounds to me that it is merely providing an option to fulfil your obligation.