Not good if the boot is not supporting your ankle in the right place. There must be a way to realign your foot in the boot so that it supports without twisting or applying pressure where you don't want it. They do have pads and inserts that change the angle and position of your foot within the boot. I find with mine I sometimes have to mess with it a lot so that it is comfortable both upright and horizontal.
I found the worse pressure sore area was the second strap from the toe, in the apex of the boot over my foot. That one keeps my heel in the boot but also rubs on the top of my foot. The one over the toes can't be tightened either due to it pressing on my broken toe.
How long have you been in a cast? After a couple of weeks the ligaments and tendons tend to tighten up. It's no wonder you're finding walking something of an effort! It'll get better. Once you regain mobility in your ankle, you ought to be able to build muscle strength back up quite quickly.
I have had a cast for a week and then the boot for six weeks so far.
I had a check up yesterday and the consultant didn't offer much more then 'carry on as you were'. I now have another six weeks in the boot but also a physiotherapy appointment in the pipeline.
I am (still) removing the boot for an hour or two, as I have been doing for the last three weeks, to move and flex my foot.
The flexibility is improving but I don't want to work too much on the strength without a Physio's advice in case I cause any damage. In the meantime I need to be careful of over stressing my other leg.
The best advance so far is being able to stand on both legs, with the boot on, as it not only gains me a leg back but also both arms so I can I hobble and shuffle while carrying tea across the room.