@geekish - I had a full blown serious stress fracture in my heel, also from just running and training for marathons.
My heel fracture became serious because I ignored it and just tried to ice it. So it doesn't sound like you have the same fracture severity as me.
I had to wear my cast boot walker (and same as you, I just had to wear the boot and did not need a cast) for 8 weeks! I was not allowed any weight bearing activity that entire time so I would ride a bike at the gym with my boot on!
One of the great things about my boot was it was an air cast boot, so I could pump air into it and the air would surround the back of my foot and up to a couple inches above my ankle. It had a deflation nozzle if too much air was pumped or it became uncomfortable.
But most of the time the air just added pressure which felt good and seemed to stabilize my foot even more.
The best news - after 8 weeks of the boot, I went back to running and my heel hasn't hurt since and I have already run one half marathon!